diff --git a/pkg/storage/mongo/mongo.go b/pkg/storage/mongo/mongo.go
index 721929ff..ff379de5 100644
--- a/pkg/storage/mongo/mongo.go
+++ b/pkg/storage/mongo/mongo.go
@@ -62,7 +62,6 @@ func (m *ModuleStore) initDatabase() error {
 	index := mgo.Index{
 		Key:        []string{"base_url", "module", "version"},
 		Unique:     true,
-		DropDups:   true,
 		Background: true,
 		Sparse:     true,
 	}
diff --git a/pkg/storage/mongo/saver.go b/pkg/storage/mongo/saver.go
index dad3cd7b..3eb45f4c 100644
--- a/pkg/storage/mongo/saver.go
+++ b/pkg/storage/mongo/saver.go
@@ -16,6 +16,14 @@ func (s *ModuleStore) Save(ctx context.Context, module, version string, mod []by
 	ctx, span := observ.StartSpan(ctx, op.String())
 	defer span.End()
 
+	exists, err := s.Exists(ctx, module, version)
+	if err != nil {
+		return errors.E(op, err, errors.M(module), errors.V(version))
+	}
+	if exists {
+		return errors.E(op, "already exists", errors.M(module), errors.V(version), errors.KindAlreadyExists)
+	}
+
 	zipName := s.gridFileName(module, version)
 	fs := s.s.DB(s.d).GridFS("fs")
 	f, err := fs.Create(zipName)