diff --git a/CHANGELOG_PENDING.md b/CHANGELOG_PENDING.md
index 22416fc3f..33fc24eaa 100644
--- a/CHANGELOG_PENDING.md
+++ b/CHANGELOG_PENDING.md
@@ -26,4 +26,5 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 
 ### BUG FIXES
 
+- [blockchain/v2] \#5499 Fix "duplicate block enqueued by processor" panic (@melekes)
 - [abci/grpc] \#5520 Return async responses in order, to avoid mempool panics. (@erikgrinaker)
diff --git a/blockchain/v2/processor.go b/blockchain/v2/processor.go
index d4b2c637b..f9036f3b9 100644
--- a/blockchain/v2/processor.go
+++ b/blockchain/v2/processor.go
@@ -17,6 +17,11 @@ type pcBlockVerificationFailure struct {
 	secondPeerID p2p.ID
 }
 
+func (e pcBlockVerificationFailure) String() string {
+	return fmt.Sprintf("pcBlockVerificationFailure{%d 1st peer: %v, 2nd peer: %v}",
+		e.height, e.firstPeerID, e.secondPeerID)
+}
+
 // successful block execution
 type pcBlockProcessed struct {
 	priorityNormal
@@ -24,6 +29,10 @@ type pcBlockProcessed struct {
 	peerID p2p.ID
 }
 
+func (e pcBlockProcessed) String() string {
+	return fmt.Sprintf("pcBlockProcessed{%d peer: %v}", e.height, e.peerID)
+}
+
 // processor has finished
 type pcFinished struct {
 	priorityNormal
@@ -87,9 +96,12 @@ func (state *pcState) synced() bool {
 }
 
 func (state *pcState) enqueue(peerID p2p.ID, block *types.Block, height int64) {
-	if _, ok := state.queue[height]; ok {
-		panic("duplicate block enqueued by processor")
+	if item, ok := state.queue[height]; ok {
+		panic(fmt.Sprintf(
+			"duplicate block %d (%X) enqueued by processor (sent by %v; existing block %X from %v)",
+			height, block.Hash(), peerID, item.block.Hash(), item.peerID))
 	}
+
 	state.queue[height] = queueItem{block: block, peerID: peerID}
 }
 
@@ -145,16 +157,20 @@ func (state *pcState) handle(event Event) (Event, error) {
 			}
 			return noOp, nil
 		}
-		first, second := firstItem.block, secondItem.block
 
-		firstParts := first.MakePartSet(types.BlockPartSizeBytes)
-		firstPartSetHeader := firstParts.Header()
-		firstID := types.BlockID{Hash: first.Hash(), PartSetHeader: firstPartSetHeader}
+		var (
+			first, second = firstItem.block, secondItem.block
+			firstParts    = first.MakePartSet(types.BlockPartSizeBytes)
+			firstID       = types.BlockID{Hash: first.Hash(), PartSetHeader: firstParts.Header()}
+		)
 
+		// verify if +second+ last commit "confirms" +first+ block
 		err = state.context.verifyCommit(tmState.ChainID, firstID, first.Height, second.LastCommit)
 		if err != nil {
 			state.purgePeer(firstItem.peerID)
-			state.purgePeer(secondItem.peerID)
+			if firstItem.peerID != secondItem.peerID {
+				state.purgePeer(secondItem.peerID)
+			}
 			return pcBlockVerificationFailure{
 					height: first.Height, firstPeerID: firstItem.peerID, secondPeerID: secondItem.peerID},
 				nil
@@ -170,7 +186,6 @@ func (state *pcState) handle(event Event) (Event, error) {
 		state.blocksSynced++
 
 		return pcBlockProcessed{height: first.Height, peerID: firstItem.peerID}, nil
-
 	}
 
 	return noOp, nil
diff --git a/blockchain/v2/reactor.go b/blockchain/v2/reactor.go
index 6aee5078e..8cb33f542 100644
--- a/blockchain/v2/reactor.go
+++ b/blockchain/v2/reactor.go
@@ -187,7 +187,7 @@ type rTryPrunePeer struct {
 }
 
 func (e rTryPrunePeer) String() string {
-	return fmt.Sprintf(": %v", e.time)
+	return fmt.Sprintf("rTryPrunePeer{%v}", e.time)
 }
 
 // ticker event for scheduling block requests
@@ -197,7 +197,7 @@ type rTrySchedule struct {
 }
 
 func (e rTrySchedule) String() string {
-	return fmt.Sprintf(": %v", e.time)
+	return fmt.Sprintf("rTrySchedule{%v}", e.time)
 }
 
 // ticker for block processing
@@ -205,6 +205,10 @@ type rProcessBlock struct {
 	priorityNormal
 }
 
+func (e rProcessBlock) String() string {
+	return "rProcessBlock"
+}
+
 // reactor generated events based on blockchain related messages from peers:
 // blockResponse message received from a peer
 type bcBlockResponse struct {
@@ -215,6 +219,11 @@ type bcBlockResponse struct {
 	block  *types.Block
 }
 
+func (resp bcBlockResponse) String() string {
+	return fmt.Sprintf("bcBlockResponse{%d#%X (size: %d bytes) from %v at %v}",
+		resp.block.Height, resp.block.Hash(), resp.size, resp.peerID, resp.time)
+}
+
 // blockNoResponse message received from a peer
 type bcNoBlockResponse struct {
 	priorityNormal
@@ -223,6 +232,11 @@ type bcNoBlockResponse struct {
 	height int64
 }
 
+func (resp bcNoBlockResponse) String() string {
+	return fmt.Sprintf("bcNoBlockResponse{%v has no block at height %d at %v}",
+		resp.peerID, resp.height, resp.time)
+}
+
 // statusResponse message received from a peer
 type bcStatusResponse struct {
 	priorityNormal
@@ -232,12 +246,21 @@ type bcStatusResponse struct {
 	height int64
 }
 
+func (resp bcStatusResponse) String() string {
+	return fmt.Sprintf("bcStatusResponse{%v is at height %d (base: %d) at %v}",
+		resp.peerID, resp.height, resp.base, resp.time)
+}
+
 // new peer is connected
 type bcAddNewPeer struct {
 	priorityNormal
 	peerID p2p.ID
 }
 
+func (resp bcAddNewPeer) String() string {
+	return fmt.Sprintf("bcAddNewPeer{%v}", resp.peerID)
+}
+
 // existing peer is removed
 type bcRemovePeer struct {
 	priorityHigh
@@ -245,12 +268,20 @@ type bcRemovePeer struct {
 	reason interface{}
 }
 
+func (resp bcRemovePeer) String() string {
+	return fmt.Sprintf("bcRemovePeer{%v due to %v}", resp.peerID, resp.reason)
+}
+
 // resets the scheduler and processor state, e.g. following a switch from state syncing
 type bcResetState struct {
 	priorityHigh
 	state state.State
 }
 
+func (e bcResetState) String() string {
+	return fmt.Sprintf("bcResetState{%v}", e.state)
+}
+
 // Takes the channel as a parameter to avoid race conditions on r.events.
 func (r *BlockchainReactor) demux(events <-chan Event) {
 	var lastRate = 0.0
diff --git a/blockchain/v2/routine.go b/blockchain/v2/routine.go
index 40e1971fe..e4ca52add 100644
--- a/blockchain/v2/routine.go
+++ b/blockchain/v2/routine.go
@@ -2,6 +2,7 @@ package v2
 
 import (
 	"fmt"
+	"strings"
 	"sync/atomic"
 
 	"github.com/Workiva/go-datastructures/queue"
@@ -11,6 +12,8 @@ import (
 
 type handleFunc = func(event Event) (Event, error)
 
+const historySize = 25
+
 // Routine is a structure that models a finite state machine as serialized
 // stream of events processed by a handle function. This Routine structure
 // handles the concurrency and messaging guarantees. Events are sent via
@@ -21,6 +24,7 @@ type Routine struct {
 	name    string
 	handle  handleFunc
 	queue   *queue.PriorityQueue
+	history []Event
 	out     chan Event
 	fin     chan error
 	rdy     chan struct{}
@@ -34,6 +38,7 @@ func newRoutine(name string, handleFunc handleFunc, bufferSize int) *Routine {
 		name:    name,
 		handle:  handleFunc,
 		queue:   queue.NewPriorityQueue(bufferSize, true),
+		history: make([]Event, 0, historySize),
 		out:     make(chan Event, bufferSize),
 		rdy:     make(chan struct{}, 1),
 		fin:     make(chan error, 1),
@@ -53,13 +58,24 @@ func (rt *Routine) setMetrics(metrics *Metrics) {
 }
 
 func (rt *Routine) start() {
-	rt.logger.Info(fmt.Sprintf("%s: run\n", rt.name))
+	rt.logger.Info(fmt.Sprintf("%s: run", rt.name))
 	running := atomic.CompareAndSwapUint32(rt.running, uint32(0), uint32(1))
 	if !running {
 		panic(fmt.Sprintf("%s is already running", rt.name))
 	}
 	close(rt.rdy)
 	defer func() {
+		if r := recover(); r != nil {
+			var (
+				b strings.Builder
+				j int
+			)
+			for i := len(rt.history) - 1; i >= 0; i-- {
+				fmt.Fprintf(&b, "%d: %+v\n", j, rt.history[i])
+				j++
+			}
+			panic(fmt.Sprintf("%v\nlast events:\n%v", r, b.String()))
+		}
 		stopped := atomic.CompareAndSwapUint32(rt.running, uint32(1), uint32(0))
 		if !stopped {
 			panic(fmt.Sprintf("%s is failed to stop", rt.name))
@@ -82,7 +98,19 @@ func (rt *Routine) start() {
 			return
 		}
 		rt.metrics.EventsOut.With("routine", rt.name).Add(1)
-		rt.logger.Debug(fmt.Sprintf("%s: produced %T %+v\n", rt.name, oEvent, oEvent))
+		rt.logger.Debug(fmt.Sprintf("%s: produced %T %+v", rt.name, oEvent, oEvent))
+
+		// Skip rTrySchedule and rProcessBlock events as they clutter the history
+		// due to their frequency.
+		switch events[0].(type) {
+		case rTrySchedule:
+		case rProcessBlock:
+		default:
+			rt.history = append(rt.history, events[0].(Event))
+			if len(rt.history) > historySize {
+				rt.history = rt.history[1:]
+			}
+		}
 
 		rt.out <- oEvent
 	}
@@ -97,7 +125,7 @@ func (rt *Routine) send(event Event) bool {
 	err := rt.queue.Put(event)
 	if err != nil {
 		rt.metrics.EventsShed.With("routine", rt.name).Add(1)
-		rt.logger.Info(fmt.Sprintf("%s: send failed, queue was full/stopped \n", rt.name))
+		rt.logger.Error(fmt.Sprintf("%s: send failed, queue was full/stopped", rt.name))
 		return false
 	}
 
@@ -122,7 +150,7 @@ func (rt *Routine) stop() {
 		return
 	}
 
-	rt.logger.Info(fmt.Sprintf("%s: stop\n", rt.name))
+	rt.logger.Info(fmt.Sprintf("%s: stop", rt.name))
 	rt.queue.Dispose() // this should block until all queue items are free?
 }
 
diff --git a/blockchain/v2/scheduler.go b/blockchain/v2/scheduler.go
index 84dee7a53..a5ca33cd0 100644
--- a/blockchain/v2/scheduler.go
+++ b/blockchain/v2/scheduler.go
@@ -2,6 +2,7 @@ package v2
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"math"
 	"sort"
@@ -18,6 +19,10 @@ type scFinishedEv struct {
 	reason string
 }
 
+func (e scFinishedEv) String() string {
+	return fmt.Sprintf("scFinishedEv{%v}", e.reason)
+}
+
 // send a blockRequest message
 type scBlockRequest struct {
 	priorityNormal
@@ -25,6 +30,10 @@ type scBlockRequest struct {
 	height int64
 }
 
+func (e scBlockRequest) String() string {
+	return fmt.Sprintf("scBlockRequest{%d from %v}", e.height, e.peerID)
+}
+
 // a block has been received and validated by the scheduler
 type scBlockReceived struct {
 	priorityNormal
@@ -32,6 +41,10 @@ type scBlockReceived struct {
 	block  *types.Block
 }
 
+func (e scBlockReceived) String() string {
+	return fmt.Sprintf("scBlockReceived{%d#%X from %v}", e.block.Height, e.block.Hash(), e.peerID)
+}
+
 // scheduler detected a peer error
 type scPeerError struct {
 	priorityHigh
@@ -40,7 +53,7 @@ type scPeerError struct {
 }
 
 func (e scPeerError) String() string {
-	return fmt.Sprintf("scPeerError - peerID %s, err %s", e.peerID, e.reason)
+	return fmt.Sprintf("scPeerError{%v errored with %v}", e.peerID, e.reason)
 }
 
 // scheduler removed a set of peers (timed out or slow peer)
@@ -49,6 +62,10 @@ type scPeersPruned struct {
 	peers []p2p.ID
 }
 
+func (e scPeersPruned) String() string {
+	return fmt.Sprintf("scPeersPruned{%v}", e.peers)
+}
+
 // XXX: make this fatal?
 // scheduler encountered a fatal error
 type scSchedulerFail struct {
@@ -56,6 +73,10 @@ type scSchedulerFail struct {
 	reason error
 }
 
+func (e scSchedulerFail) String() string {
+	return fmt.Sprintf("scSchedulerFail{%v}", e.reason)
+}
+
 type blockState int
 
 const (
@@ -295,6 +316,9 @@ func (sc *scheduler) setPeerRange(peerID p2p.ID, base int64, height int64) error
 	}
 
 	if base > height {
+		if err := sc.removePeer(peerID); err != nil {
+			return err
+		}
 		return fmt.Errorf("cannot set peer base higher than its height")
 	}
 
@@ -418,7 +442,7 @@ func (sc *scheduler) markProcessed(height int64) error {
 		return fmt.Errorf("cannot mark height %d received from block state %s", height, state)
 	}
 
-	sc.height++
+	sc.height = height + 1
 	delete(sc.receivedBlocks, height)
 	delete(sc.blockStates, height)
 	sc.addNewBlocks()
@@ -532,14 +556,12 @@ func (sc *scheduler) handleBlockResponse(event bcBlockResponse) (Event, error) {
 }
 
 func (sc *scheduler) handleNoBlockResponse(event bcNoBlockResponse) (Event, error) {
-	if len(sc.peers) == 0 {
-		return noOp, nil
-	}
-
+	// No such peer or peer was removed.
 	peer, ok := sc.peers[event.peerID]
 	if !ok || peer.state == peerStateRemoved {
 		return noOp, nil
 	}
+
 	// The peer may have been just removed due to errors, low speed or timeouts.
 	_ = sc.removePeer(event.peerID)
 
@@ -550,8 +572,9 @@ func (sc *scheduler) handleNoBlockResponse(event bcNoBlockResponse) (Event, erro
 
 func (sc *scheduler) handleBlockProcessed(event pcBlockProcessed) (Event, error) {
 	if event.height != sc.height {
-		panic(fmt.Sprintf("processed height %d but expected height %d", event.height, sc.height))
+		panic(fmt.Sprintf("processed height %d, but expected height %d", event.height, sc.height))
 	}
+
 	err := sc.markProcessed(event.height)
 	if err != nil {
 		// It is possible that a peer error or timeout is handled after the processor
@@ -601,11 +624,13 @@ func (sc *scheduler) handleRemovePeer(event bcRemovePeer) (Event, error) {
 	if sc.allBlocksProcessed() {
 		return scFinishedEv{reason: "removed peer"}, nil
 	}
-	return noOp, nil
+
+	// Return scPeerError so the peer (and all associated blocks) is removed from
+	// the processor.
+	return scPeerError{peerID: event.peerID, reason: errors.New("peer was stopped")}, nil
 }
 
 func (sc *scheduler) handleTryPrunePeer(event rTryPrunePeer) (Event, error) {
-
 	// Check behavior of peer responsible to deliver block at sc.height.
 	timeHeightAsked, ok := sc.pendingTime[sc.height]
 	if ok && time.Since(timeHeightAsked) > sc.peerTimeout {
diff --git a/blockchain/v2/scheduler_test.go b/blockchain/v2/scheduler_test.go
index a4636d954..fce0c0563 100644
--- a/blockchain/v2/scheduler_test.go
+++ b/blockchain/v2/scheduler_test.go
@@ -586,8 +586,7 @@ func TestScSetPeerRange(t *testing.T) {
 				allB:  []int64{1, 2, 3, 4}},
 			args: args{peerID: "P1", base: 6, height: 5},
 			wantFields: scTestParams{
-				peers: map[string]*scPeer{"P1": {height: 4, state: peerStateReady}},
-				allB:  []int64{1, 2, 3, 4}},
+				peers: map[string]*scPeer{"P1": {height: 4, state: peerStateRemoved}}},
 			wantErr: true,
 		},
 		{