output unique keys

README.md

@@ -1038,14 +1038,18 @@ result = lo.Splice([]string{"a", "b"}, 42, "1", "2")
 
 ### Keys
 
-Creates an array of the map keys.
+Creates an array of the unique map keys.
+(Note: The order of the keys is not guaranteed to be the same as the order returned by the map, so can sort them if needed.)
 
 ```go
 keys := lo.Keys(map[string]int{"foo": 1, "bar": 2})
 // []string{"foo", "bar"}
 
 keys := lo.Keys(map[string]int{"foo": 1, "bar": 2}, map[string]int{"baz": 3})
 // []string{"foo", "bar", "baz"}
+
+keys := lo.Keys(map[string]int{"foo": 1, "bar": 2}, map[string]int{"bar": 3})
+// []string{"foo", "bar"}
 ```
 
 [[play](https://go.dev/play/p/Uu11fHASqrU)]

map.go

@@ -3,12 +3,18 @@ package lo
 // Keys creates an array of the map keys.
 // Play: https://go.dev/play/p/Uu11fHASqrU
 func Keys[K comparable, V any](in ...map[K]V) []K {
+	seen := make(map[K]bool)
 	result := make([]K, 0)
+
 	for i := range in {
 		for k := range in[i] {
-			result = append(result, k)
+			if _, exists := seen[k]; !exists {
+				seen[k] = true
+				result = append(result, k)
+			}
 		}
 	}
+
 	return result
 }
 

map_test.go

@@ -24,12 +24,12 @@ func TestKeys(t *testing.T) {
 	sort.Strings(r3)
 	is.Equal(r3, []string{"bar", "baz", "foo"})
 
-	r4 := Keys[int, int]()
-	is.Equal(r4, []int{})
+	r4 := Keys[string, int]()
+	is.Equal(r4, []string{})
 
 	r5 := Keys(map[string]int{"foo": 1, "bar": 2}, map[string]int{"bar": 3})
 	sort.Strings(r5)
-	is.Equal(r5, []string{"bar", "bar", "foo"})
+	is.Equal(r5, []string{"bar", "foo"})
 }
 
 func TestHasKey(t *testing.T) {