Some apps require that your company and/or contacts must be unique and do not allow duplicates. In other cases you may request custom matching to existing data in your new app. In either case, you can expect Import2 to:
Not overwrite or update fields
Not overwrite or update record links (i.e. contact/deal, company/contact, etc)
Add new activities, notes, etc. to the record's timeline/history
We recommend that you consult the App-Specific FAQs to confirm if your new app has requirements around duplicate data, and understand the criteria for how a match is found.