diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
index 7355e1dba1..a755aee55e 100644
--- a/src/backend/access/nbtree/nbtinsert.c
+++ b/src/backend/access/nbtree/nbtinsert.c
@@ -1164,10 +1164,29 @@ _bt_insertonpg(Relation rel,
 		 * its post-split version is treated as an extra step in either the
 		 * insert or page split critical section.
 		 */
-		Assert(isleaf && !ItemIdIsDead(itemid));
-		Assert(itup_key->heapkeyspace && itup_key->allequalimage);
+		Assert(isleaf && itup_key->heapkeyspace && itup_key->allequalimage);
 		oposting = (IndexTuple) PageGetItem(page, itemid);
 
+		/*
+		 * postingoff value comes from earlier call to _bt_binsrch_posting().
+		 * Its binary search might think that a plain tuple must be a posting
+		 * list tuple that needs to be split.  This can happen with corruption
+		 * involving an existing plain tuple that is a duplicate of the new
+		 * item, up to and including its table TID.  Check for that here in
+		 * passing.
+		 *
+		 * Also verify that our caller has made sure that the existing posting
+		 * list tuple does not have its LP_DEAD bit set.
+		 */
+		if (!BTreeTupleIsPosting(oposting) || ItemIdIsDead(itemid))
+			ereport(ERROR,
+					(errcode(ERRCODE_INDEX_CORRUPTED),
+					 errmsg_internal("table tid from new index tuple (%u,%u) overlaps with invalid duplicate tuple at offset %u of block %u in index \"%s\"",
+									 ItemPointerGetBlockNumber(&itup->t_tid),
+									 ItemPointerGetOffsetNumber(&itup->t_tid),
+									 BufferGetBlockNumber(buf), newitemoff,
+									 RelationGetRelationName(rel))));
+
 		/* use a mutable copy of itup as our itup from here on */
 		origitup = itup;
 		itup = CopyIndexTuple(origitup);
diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index d1177d8772..fdf0e5654a 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -526,7 +526,24 @@ _bt_binsrch_insert(Relation rel, BTInsertState insertstate)
 		 * infrequently.
 		 */
 		if (unlikely(result == 0 && key->scantid != NULL))
+		{
+			/*
+			 * postingoff should never be set more than once per leaf page
+			 * binary search.  That would mean that there are duplicate table
+			 * TIDs in the index, which is never okay.  Check for that here.
+			 */
+			if (insertstate->postingoff != 0)
+				ereport(ERROR,
+						(errcode(ERRCODE_INDEX_CORRUPTED),
+						 errmsg_internal("table tid from new index tuple (%u,%u) cannot find insert offset between offsets %u and %u of block %u in index \"%s\"",
+										 ItemPointerGetBlockNumber(key->scantid),
+										 ItemPointerGetOffsetNumber(key->scantid),
+										 low, stricthigh,
+										 BufferGetBlockNumber(insertstate->buf),
+										 RelationGetRelationName(rel))));
+
 			insertstate->postingoff = _bt_binsrch_posting(key, page, mid);
+		}
 	}
 
 	/*