option to display all ambiguous citekeys

zotero citekeys are suffixed with a -1, -2 etc. if an author publised
more than two items in one year. however, using quick-copy to get the
citekey, the suffix is ommited. moreover, when adding new items, the
suffixes can change. activating the new option finds all ambiguous
citekeys and echoes them. like that, one can check which citation was
actually meant.