What version of Act are you running? You can perform a duplicate check during the import of external data. The following Knowledgebase article has further details on this: http://kb.act.com/app/answers/detail/a_id/38088
If you want to do a duplicate check without any importing, you could export your Act contacts to excel from the contact list view, include your external data, then check for duplicates using features in Excel.
What version of Act are you running? You can perform a duplicate check during the import of external data. The following Knowledgebase article has further details on this: http://kb.act.com/app/answers/detail/a_id/38088
If you want to do a duplicate check without any importing, you could export your Act contacts to excel from the contact list view, include your external data, then check for duplicates using features in Excel.