--- a/jdmarker.c
+++ b/jdmarker.c
@@ -342,6 +342,12 @@ get_sos (j_decompress_ptr cinfo)
     
     TRACEMS3(cinfo, 1, JTRC_SOS_COMPONENT, cc,
 	     compptr->dc_tbl_no, compptr->ac_tbl_no);
+
+    /* This CSi (cc) should differ from the previous CSi */
+    for (ci = 0; ci < i; ci++) {
+      if (cinfo->cur_comp_info[ci] == compptr)
+        ERREXIT1(cinfo, JERR_BAD_COMPONENT_ID, cc);
+    }
   }
 
   /* Collect the additional scan parameters Ss, Se, Ah/Al. */