Index: src/insert.c
==================================================================
--- src/insert.c
+++ src/insert.c
@@ -1580,10 +1580,12 @@
         }
         if( pTrigger || sqlite3FkRequired(pParse, pTab, 0, 0) ){
           sqlite3MultiWrite(pParse);
           sqlite3GenerateRowDelete(pParse, pTab, pTrigger, iDataCur, iIdxCur,
                                    regNewData, 1, 0, OE_Replace, 1, -1);
+          sqlite3VdbeAddOp3(v, OP_NotExists, iDataCur, addrRowidOk, regNewData);
+          sqlite3RowidConstraint(pParse, OE_Abort, pTab);
         }else{
 #ifdef SQLITE_ENABLE_PREUPDATE_HOOK
           assert( HasRowid(pTab) );
           /* This OP_Delete opcode fires the pre-update-hook only. It does
           ** not modify the b-tree. It is more efficient to let the coming
@@ -1827,20 +1829,27 @@
         sqlite3VdbeGoto(v, ignoreDest);
         break;
       }
       default: {
         Trigger *pTrigger = 0;
+        int bRetryConstraintCheck = 0;
         assert( onError==OE_Replace );
         if( db->flags&SQLITE_RecTriggers ){
           pTrigger = sqlite3TriggersExist(pParse, pTab, TK_DELETE, 0, 0);
         }
         if( pTrigger || sqlite3FkRequired(pParse, pTab, 0, 0) ){
           sqlite3MultiWrite(pParse);
+          bRetryConstraintCheck = 1;
         }
         sqlite3GenerateRowDelete(pParse, pTab, pTrigger, iDataCur, iIdxCur,
             regR, nPkField, 0, OE_Replace,
             (pIdx==pPk ? ONEPASS_SINGLE : ONEPASS_OFF), iThisCur);
+        if( bRetryConstraintCheck ){
+          sqlite3VdbeAddOp4Int(v, OP_NoConflict, iThisCur, addrUniqueOk,
+                               regIdx, pIdx->nKeyCol); VdbeCoverage(v);
+          sqlite3UniqueConstraint(pParse, OE_Abort, pIdx);
+        }
         seenReplace = 1;
         break;
       }
     }
     if( pUpIdx==pIdx ){

Index: test/insert.test
==================================================================
--- test/insert.test
+++ test/insert.test
@@ -1,6 +1,6 @@
-# 2001 September 15
+# 2001-09-15
 #
 # The author disclaims copyright to this source code.  In place of
 # a legal notice, here is a blessing:
 #
 #    May you do good and not evil.
@@ -9,11 +9,10 @@
 #
 #***********************************************************************
 # This file implements regression tests for SQLite library.  The
 # focus of this file is testing the INSERT statement.
 #
-# $Id: insert.test,v 1.31 2007/04/05 11:25:59 drh Exp $
 
 set testdir [file dirname $argv0]
 source $testdir/tester.tcl
 
 # Try to insert into a non-existant table.
@@ -456,11 +455,11 @@
   CREATE TABLE t14(x INTEGER PRIMARY KEY);
   INSERT INTO t14 VALUES(CASE WHEN 1 THEN null END);
   SELECT x FROM t14;
 } {1}
 
-integrity_check insert-99.0
+integrity_check insert-14.2
 
 # 2019-08-12.
 #
 do_execsql_test insert-15.1 {
   DROP TABLE IF EXISTS t1;
@@ -473,7 +472,55 @@
   INSERT INTO t2 VALUES(4, randomblob(33000));
   REPLACE INTO t1 SELECT a, b FROM t2;
   SELECT a, length(b) FROM t1;
 } {4 33000}
 
+# 2019-10-16
+# ticket https://www.sqlite.org/src/info/a8a4847a2d96f5de
+# On a REPLACE INTO, if an AFTER trigger adds back the conflicting
+# row, you can end up with the wrong number of rows in an index.
+#
+db close
+sqlite3 db :memory:
+do_catchsql_test insert-16.1 {
+  PRAGMA recursive_triggers = true;
+  CREATE TABLE t0(c0,c1);
+  CREATE UNIQUE INDEX i0 ON t0(c0);
+  INSERT INTO t0(c0,c1) VALUES(123,1);
+  CREATE TRIGGER tr0 AFTER DELETE ON t0
+  BEGIN
+    INSERT INTO t0 VALUES(123,2);
+  END;
+  REPLACE INTO t0(c0,c1) VALUES(123,3);
+} {1 {UNIQUE constraint failed: t0.c0}}
+do_execsql_test insert-16.2 {
+  SELECT * FROM t0;
+} {123 1}
+integrity_check insert-16.3
+do_catchsql_test insert-16.4 {
+  CREATE TABLE t1(a INTEGER PRIMARY KEY, b);
+  CREATE INDEX t1b ON t1(b);
+  INSERT INTO t1 VALUES(1, 'one');
+  CREATE TRIGGER tr3 AFTER DELETE ON t1 BEGIN
+    INSERT INTO t1 VALUES(1, 'three');
+  END;
+  REPLACE INTO t1 VALUES(1, 'two');
+} {1 {UNIQUE constraint failed: t1.a}}
+integrity_check insert-16.5
+do_catchsql_test insert-16.6 {
+  PRAGMA foreign_keys = 1;
+  CREATE TABLE p1(a, b UNIQUE);
+  CREATE TABLE c1(c, d REFERENCES p1(b) ON DELETE CASCADE);
+  CREATE TRIGGER tr6 AFTER DELETE ON c1 BEGIN
+    INSERT INTO p1 VALUES(4, 1);
+  END;
+  INSERT INTO p1 VALUES(1, 1);
+  INSERT INTO c1 VALUES(2, 1);
+  REPLACE INTO p1 VALUES(3, 1);
+} {1 {UNIQUE constraint failed: p1.b}}
+integrity_check insert-16.7
+
+
+
+  
 
 finish_test