Add SliceFindDuplicates function

slice.go

@@ -54,3 +54,23 @@ func SliceMapFunc[S ~[]E, E any](s S, f func(E, int) E) S {
 	}
 	return r
 }
+
+// SliceFindDuplicates takes a slice of comparable elements and returns a new slice containing
+// only the elements that appear more than once in the input slice. Since a map is used
+// internally to track duplicates, the order of the elements in the output slice is not guaranteed.
+func SliceFindDuplicates[S ~[]E, E comparable](s S) S {
+	elementCount := make(map[E]int)
+	duplicates := []E{}
+
+	for _, element := range s {
+		elementCount[element]++
+	}
+
+	for element, count := range elementCount {
+		if count > 1 {
+			duplicates = append(duplicates, element)
+		}
+	}
+
+	return duplicates
+}

slice_test.go

@@ -90,3 +90,13 @@ func (s *SliceFuncs) TestSliceMapFunc() {
 		return strings.TrimSpace(v)
 	}), "SliceMapFunc should return {\"a\", \"b\", \"c\"}")
 }
+
+func (s *SliceFuncs) TestSliceFindDuplicates() {
+	a := []string{"foo", "bar", "bar", "foo", "roll", "roll"}
+	b := []string{"apple", "book", "clock", "duck", "escape", "field"}
+	c := []string{"foo", "bar", "roll", "roll", "desk"}
+
+	require.ElementsMatch(s.T(), []string{"foo", "bar", "roll"}, SliceFindDuplicates(a), "SliceFindDuplicates should return elements: {foo, bar, roll}")
+	require.ElementsMatch(s.T(), []string{}, SliceFindDuplicates(b), "SliceFindDuplicates should return elements: {}")
+	require.ElementsMatch(s.T(), []string{"roll"}, SliceFindDuplicates(c), "SliceFindDuplicates should return elements: {roll}")
+}