Index: src/select.c
==================================================================
--- src/select.c
+++ src/select.c
@@ -4072,11 +4072,11 @@
   return 1;
 }
 #endif /* !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW) */
 
 /*
-** A structure to keep track of all of the column values that fixed to
+** A structure to keep track of all of the column values that are fixed to
 ** a known value due to WHERE clause constraints of the form COLUMN=VALUE.
 */
 typedef struct WhereConst WhereConst;
 struct WhereConst {
   Parse *pParse;   /* Parsing context */
@@ -4084,17 +4084,32 @@
   int nChng;       /* Number of times a constant is propagated */
   Expr **apExpr;   /* [i*2] is COLUMN and [i*2+1] is VALUE */
 };
 
 /*
-** Add a new entry to the pConst object
+** Add a new entry to the pConst object.  Except, do not add duplicate
+** pColumn entires.
 */
 static void constInsert(
-  WhereConst *pConst,
-  Expr *pColumn,
-  Expr *pValue
+  WhereConst *pConst,      /* The WhereConst into which we are inserting */
+  Expr *pColumn,           /* The COLUMN part of the constraint */
+  Expr *pValue             /* The VALUE part of the constraint */
 ){
+  int i;
+  assert( pColumn->op==TK_COLUMN );
+
+  /* 2018-10-25 ticket [cf5ed20f]
+  ** Make sure the same pColumn is not inserted more than once */
+  for(i=0; i<pConst->nConst; i++){
+    const Expr *pExpr = pConst->apExpr[i*2];
+    assert( pExpr->op==TK_COLUMN );
+    if( pExpr->iTable==pColumn->iTable
+     && pExpr->iColumn==pColumn->iColumn
+    ){
+      return;  /* Already present.  Return without doing anything. */
+    }
+  }
 
   pConst->nConst++;
   pConst->apExpr = sqlite3DbReallocOrFree(pConst->pParse->db, pConst->apExpr,
                          pConst->nConst*2*sizeof(Expr*));
   if( pConst->apExpr==0 ){

Index: test/whereL.test
==================================================================
--- test/whereL.test
+++ test/whereL.test
@@ -108,7 +108,18 @@
          B
    WHERE A.id='1'
      AND A.id=subq.yy
      AND B.id=subq.zz;
 } {1}  
+
+# 2018-10-25: Ticket [cf5ed20f]
+# Incorrect join result with duplicate WHERE clause constraint.
+#
+do_execsql_test 400 {
+  CREATE TABLE x(a, b, c);
+  CREATE TABLE y(a, b);
+  INSERT INTO x VALUES (1, 0, 1);
+  INSERT INTO y VALUES (1, 2);
+  SELECT x.a FROM x JOIN y ON x.c = y.a WHERE x.b = 1 AND x.b = 1;
+} {}
 
 finish_test