When running a Match task the following warning will be returned if there are multiple target accounts with the same 'displayName'.
Multiple target accounts with the attribute 'displayName' that is equal to 'John Doe' were found
When there are multiple accounts with the same value for the attribute you are matching on you can choose another attribute when creating the match task or use a mapping file.
The mapping file will allow you to specify the source and target UPN to match against and further details can be found here.
