Hi
Update:
Here comes a suggestion. To your current records, add a column, call it LeadID and gen it in a way that its not duplicated and will still identify your forthcoming records and we can then have a script or automation to MERGE it upon criteria that, believe me, will be well thought through. I've done this, I know how lead records without ID mess up your contacts.
I can take lead in this journey.
Update Finish:
Original Bid:
What is needed the most is, will the new data have primary key. with primary key duplication is easy to manage. I know from experience, duplication comes as a nightmare, with field duplicate, record duplication, primary key poisoning making it all manual. I will be in a better position to pitch once I have a gist of the current and forthcoming data.
Regards
Umer
p.s I am currently handling a similar situation.