The first thing you need to do is to use a superkey instead of an autonumber primary key.
A superkey is a combination of various data elements that will be the same no mater where the data is first entered. For example, if the database is being used to enter customer orders in the field, the superkey might be a combination of the salesperson's name, the customer name or number, the date of the order, AND an autonumber field (in case the sales person has many orders from the same customer.)
Then to uplaod this new data, create a find unmatched query that uses the superkey field, and then do an append query for all records that don't have matches.
You can make this process fully automated, or have the salesperson, in this case, start the process and approve each transaction.
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