It would be nice if the duplicate check used in cataloging and some other places could also take into account the 001 + 003 fields. When working with a union catalog this would be the best check to perform to make sure any real duplicates are found. Not sure if this needs to be optional. Bug 15248 asked about making the checks configurable, but I am not sure what a good syntax here could look like. Having several preconfigured options in a (multi-)select preference might be easier.