Thanks for your answer. Sorry I need to clarify the scenario. We have an supplier catalog (lets call it "Previously Existing Articles") which contains articles from an previous system. We do not want to merge/import new articles into the master catalog if they exist in Previously Existing Articles catalog. In order to compare if 2 articles are equal GTIN is used.
So we want to be able to ignore the merge/import of an article if it is found in the Previously Existing Articles by a customization. Is this something you have any experience in doing?
It just came to my mind that we have an import function, which looks up an item by their GTIN. It is even possible to give it a specific catalog.
Maybe you can use this function to skip the import if the GTIN is found in your "previous items" catalog.
However, since this is not really how this function was intended to work you should do some performance tests, because this seems like an expensive operation.