It may be an over simplification, but here is a rule with downloading that always works.

The data, whether it's a client's record, an account record, or an investment, is either already in Advisors Assistant, or it's not.

Throughout the download process you'll see lists and the available of list.  You'll also see buttons for either adding data or matching data.

Add Buttons And Match Buttons

When you've determined that the information is not in Advisors Assistant, just use the Add Button.

When you've determined that the information is already in Advisors Assistant, just use the Match Button.