diff --git a/core/hash-sets/hash-sets-tests.factor b/core/hash-sets/hash-sets-tests.factor
index ca995a38e6..2432d9c5bf 100644
--- a/core/hash-sets/hash-sets-tests.factor
+++ b/core/hash-sets/hash-sets-tests.factor
@@ -34,3 +34,7 @@ IN: hash-sets.tests
 
 [ t ] [ HS{ } null? ] unit-test
 [ f ] [ HS{ 1 } null? ] unit-test
+
+{ { } } [ { 1 2 3 } duplicates ] unit-test
+{ f } [ { 1 2 3 } >hash-set duplicates ] unit-test
+{ { 1 } } [ { 1 2 1 } duplicates ] unit-test
diff --git a/core/hash-sets/hash-sets.factor b/core/hash-sets/hash-sets.factor
index 6e624dec70..c009593c58 100644
--- a/core/hash-sets/hash-sets.factor
+++ b/core/hash-sets/hash-sets.factor
@@ -31,7 +31,7 @@ M: sequence fast-set >hash-set ;
 M: f fast-set drop H{ } clone hash-set boa ;
 
 M: sequence duplicates
-    f fast-set [ [ in? ] [ adjoin ] 2bi ] curry filter ;
+    f fast-set [ ?adjoin not ] curry filter ;
 
 <PRIVATE