diff --git a/src/eval.c b/src/eval.c
index fa9966e..9a81b4f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -10191,7 +10191,7 @@
 			EMSG2(_("E737: Key already exists: %s"), hi2->hi_key);
 			break;
 		    }
-		    else if (*action == 'f')
+		    else if (*action == 'f' && HI2DI(hi2) != di1)
 		    {
 			clear_tv(&di1->di_tv);
 			copy_tv(&HI2DI(hi2)->di_tv, &di1->di_tv);
diff --git a/src/testdir/test55.in b/src/testdir/test55.in
index 0cac348..cf85f57 100644
--- a/src/testdir/test55.in
+++ b/src/testdir/test55.in
@@ -352,6 +352,24 @@
 :let dict4copy = deepcopy(dict4)
 :$put =(l == lcopy)
 :$put =(dict4 == dict4copy)
+:"
+:" Pass the same List to extend()
+:let l = [1, 2, 3, 4, 5]
+:call extend(l, l)
+:$put =string(l)
+:"
+:" Pass the same Dict to extend()
+:let d = { 'a': {'b': 'B'}}
+:call extend(d, d)
+:$put =string(d)
+:"
+:" Pass the same Dict to extend() with "error"
+:try
+:  call extend(d, d, "error")
+:catch
+:  $put =v:exception[:15] . v:exception[-1:-1]
+:endtry
+:$put =string(d)
 :endfun
 :"
 :call Test(1, 2, [3, 4], {5: 6})  " This may take a while
diff --git a/src/testdir/test55.ok b/src/testdir/test55.ok
index 6e40235..fde085c 100644
--- a/src/testdir/test55.ok
+++ b/src/testdir/test55.ok
@@ -111,3 +111,7 @@
 0
 1
 1
+[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
+{'a': {'b': 'B'}}
+Vim(call):E737: a
+{'a': {'b': 'B'}}
diff --git a/src/version.c b/src/version.c
index 317a2d1..5b4bbfc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -715,6 +715,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    516,
+/**/
     515,
 /**/
     514,