External ID is unique forever. There is another field that I can't remember what it's called that you can provide that's unique per upload.

thanks for your reply.. that creates a difficult situation for one offs if there is some automated system in place for regular SO creations. 😞 I guess I need to find out how they are currently generating the external ids off system.

IIRC you can map an arbitrary unique value to Internal Id and it will use that to differentiate records

That could fail if it's numeric only and within the range of numbers valid for the record for you are uploading. Otherwise, I suppose i don't see anything wrong with it but the idea does make me nervous for some reason.