Feature to prevent duplicate import?

Issue Summary

Hello,

Thank you for the useful biblio add-in, which is proving very helpful to me. The only recurring issue I have is that it is possible to import the same article, with the same PubMed ID, twice. It would be useful if the user were prompted if that PMID is already present, and if they choose to import, the record should probably be over-written, not duplicated. Or else a simple checkbox would be handy, that would prevent import if that PMID is there already.

Thanks,

Jon