You can send Evercontact contact updates directly into any address book you'd like by choosing as seen below in Outlook 2010 (in 2007 this is in Menu > Tools > Options > Evercontact). 

If you choose to create and maintain a separate address book for Evercontact and leave the contacts there, then you're all set.  HOWEVER, if you choose to manually add these contacts to another address book as seen below...

... then we encourage you to "copy and paste" these contacts, and not "drag and drop".  Both actions will update and dedupe that contact in your other Outlook address book as seen below.

...but "drag and drop" moves it without leaving a copy in the Evercontact address book--- meaning the plugin will continue to propose this as a new contact as it does not see it within your Evercontact address book.