It shouldn’t need to be “unique” as Endnote looks at both Author year and record number (but it can be any Author, not just the first author, which is where it sometimes glitches). But I find page nos, which is an alternate option for the temporary citation in Endote’s preferences works pretty well.
I have also transferred/copied the Record number to the lable field and added a prefix and two XX as a suffix (using the move change option) when I joined two libraries, each with a unique prefix, and the label field can also be used as a identifier instead of record number (or page number). – the trick is maintaining something in the field as I add records, but I think endnote may default to record numbers if the lable field is empty.