I was searching forum for this idea, maybe I was sloppy and didn't find anything.
I guess by now Numista site is ready for some automatic checking if coin that is about to be added OR SIMILAR ONE already exists in database or maybe between not validated ones yet. I guess in the beginning checks may be made with using field "Reference number" (if it is the same) and later on all the way, checks may become "smarter" eg. checking if "Name of the coin", "Currency", "Metal", "Weight", "Diameter" and "Thickness" and so on have very similar entries (how similar is the different question). The same tool could be used for easier referencing to other coins, without the need to actually search for them.
Methinks that if one's competent enough to find out what it's numismatic reference number is, they should also know how to search the catalogue well enough to see if the coin's already there. Still a good failsafe to be implemented IMO, though.