diff --git a/storage/filesystem.go b/storage/filesystem.go
index 3ad71fd..f9bf934 100644
--- a/storage/filesystem.go
+++ b/storage/filesystem.go
@@ -178,7 +178,7 @@ func (b *filesystemBackend) GetAddressObject(ctx context.Context, path string, r
 		return nil, err
 	}
 
-	propFilter := []string{}
+	var propFilter []string
 	if req != nil && !req.AllProp {
 		propFilter = req.Props
 	}
@@ -194,10 +194,10 @@ func (b *filesystemBackend) GetAddressObject(ctx context.Context, path string, r
 	}
 
 	obj := carddav.AddressObject{
-		Path: path,
+		Path:    path,
 		ModTime: info.ModTime(),
-		ETag: etag,
-		Card: *card,
+		ETag:    etag,
+		Card:    *card,
 	}
 	return &obj, nil
 }
@@ -227,10 +227,10 @@ func (b *filesystemBackend) loadAll(ctx context.Context, propFilter []string) ([
 		}
 
 		obj := carddav.AddressObject{
-			Path: "/" + filepath.Base(filename),
+			Path:    "/" + filepath.Base(filename),
 			ModTime: info.ModTime(),
-			ETag: etag,
-			Card: *card,
+			ETag:    etag,
+			Card:    *card,
 		}
 		result = append(result, obj)
 		return nil
@@ -240,7 +240,7 @@ func (b *filesystemBackend) loadAll(ctx context.Context, propFilter []string) ([
 }
 
 func (b *filesystemBackend) ListAddressObjects(ctx context.Context, req *carddav.AddressDataRequest) ([]carddav.AddressObject, error) {
-	propFilter := []string{}
+	var propFilter []string
 	if req != nil && !req.AllProp {
 		propFilter = req.Props
 	}
@@ -249,7 +249,7 @@ func (b *filesystemBackend) ListAddressObjects(ctx context.Context, req *carddav
 }
 
 func (b *filesystemBackend) QueryAddressObjects(ctx context.Context, query *carddav.AddressBookQuery) ([]carddav.AddressObject, error) {
-	propFilter := []string{}
+	var propFilter []string
 	if query != nil && !query.DataRequest.AllProp {
 		propFilter = query.DataRequest.Props
 	}
@@ -262,11 +262,34 @@ func (b *filesystemBackend) QueryAddressObjects(ctx context.Context, query *card
 	return carddav.Filter(query, result)
 }
 
+func (b *filesystemBackend) hasUIDConflict(ctx context.Context, uid, path string) (bool, error) {
+	all, err := b.loadAll(ctx, nil)
+	if err != nil {
+		return false, err
+	}
+	for _, contact := range all {
+		if contact.Path != path && contact.Card.Value(vcard.FieldUID) == uid {
+			return true, nil
+		}
+	}
+
+	return false, nil
+}
+
 func (b *filesystemBackend) PutAddressObject(ctx context.Context, path string, card vcard.Card) (loc string, err error) {
 	localPath, err := b.safePath(ctx, path)
 	if err != nil {
 		return "", err
 	}
+
+	conflict, err := b.hasUIDConflict(ctx, card.Value(vcard.FieldUID), path)
+	if err != nil {
+		return "", err
+	}
+	if conflict {
+		return "", carddav.NewPreconditionError(carddav.PreconditionNoUIDConflict)
+	}
+
 	f, err := os.OpenFile(localPath, os.O_RDWR|os.O_CREATE, 0644)
 	if err != nil {
 		return "", err