Once you have imported your address book use the Tools scan for duplicates in ACT and then use the Tools Copy/move to merge these duplicates into a single record. Make sure that you do a backup beforehand.
Paul Buchtmann firstname.lastname@example.org Australia Australia's First ACT Consultant (1989)
FREE Record History to change default to Call or To-Do for ACT! 2010-2012 versions. Custom versions also available.