For versions 5.0.18+ there are two known issues that can cause multiple preferred addresses
An account can get multiple preferred addresses under the following circumstances:
1) Account has an active preferred address
2) You use ADU to upload a record with a different preferred address with blank status that matches to an existing address on the account that is not active
3) The uploaded record from step 2 is deferred to reconciliation and then matched to an account
Uploading addresses with a status other than 'A' and the preferred_address = 'Y' when the account already has an active address that was the preferred_address, you'll end up with two addresses marked as the preferred address when the uploaded address creates a new record (rather than updating an existing one), both the uploaded address and an active address.