Solutions for importing marketing lists without creating duplicate contacts?

I'm interested in solutions for importing a list of contacts to be added to a Marketing list without creating duplicate contacts in the CRM, similar to the functionality Click Dimensions offers.

http://help.clickdimensions.com/import/

We have clients using Click Dimensions; however, we also have clients with other Digital Marketing solutions that are interested in similar functionality.  Are you aware of a standalone list import tool or workflow that does not import duplicate contacts?