Add Assets to Variants

Trying to add images to variants.

The code looks like this:

var parentLink = ReferenceConverter.Service.GetContentLink(parentCode);
F parent = ContentRepository.Service.TryGet(parentLink.ToReferenceWithoutVersion(), _catalogService.Culture, out parent) ? parent.CreateWritableClone<F>() : ContentRepository.Service.GetDefault<F>(parentLink, _catalogService.Culture);
parent.CommerceMediaCollection.Add(commerceMedia);
ContentRepository.Service.Save(parent, SaveAction.Publish, AccessLevel.NoAccess);

This works well the first time, but if the import should handle a change for this article again at a later date, the code will crash.

error message:
"Violation of PRIMARY KEY constraint 'PK_ecfVersionAsset'. Cannot insert duplicate key in object 'dbo.ecfVersionAsset'. The duplicate key value is (8078274, image.jpg, a910104b-a4ee-44e8-8cd3-33950f19d9ae)." has been terminated. "

Any suggestions on what's wrong, how to get on?