If I upload a list of ASINs already on Sellics, will they override those already in the system?
Nope! If a product ASIN is already uploaded within a market, it will not be impacted by a new, duplicate ASIN record. The record will simply not load. If a separate market is chosen, then the ASIN will be uploaded and assigned under that market.
If you're loading a batch of ASINs, those which have already been loaded will be declined, while the new ASINs will go ahead and be loaded.