check same source path after clean

mmv.go

@@ -31,6 +31,14 @@ func (err *emptyPathError) Error() string {
 	return "empty path error"
 }
 
+type sameSourceError struct {
+	path string
+}
+
+func (err *sameSourceError) Error() string {
+	return fmt.Sprintf("duplicate source: %s", err.path)
+}
+
 type sameDestinationError struct {
 	path string
 }
@@ -57,6 +65,9 @@ func buildRenames(files map[string]string) ([]rename, error) {
 		if d := filepath.Clean(src); d != src {
 			delete(files, src)
 			src = d
+			if _, ok := files[src]; ok {
+				return nil, &sameSourceError{src}
+			}
 			files[src] = dst
 		}
 		if d := filepath.Clean(dst); d != dst {

mmv_test.go

@@ -186,6 +186,18 @@ func TestRename(t *testing.T) {
 				"foo": "1",
 			},
 		},
+		{
+			name: "cleaned path same source error",
+			files: map[string]string{
+				"foo":        "baz",
+				"bar/../foo": "bar",
+			},
+			contents: map[string]string{
+				"foo": "0",
+				"bar": "1",
+			},
+			err: &sameSourceError{"foo"},
+		},
 		{
 			name: "cleaned path same destination error",
 			files: map[string]string{