While still requiring a manual task pre-import, this should only take about 10/15mins. I personally would export all of my current database, strip the email field out and use that to highlight all my existing HubSpot contacts in the old CRM file via the following formula:
=IF(ISNA(VLOOKUP(A2,D : D,1,0)),“No”,“Yes”)
(edit: remove spacing on D : D, the community portal is converting the formula into emoji’s!)
With A2 being the email address of a contact in the CRM file, and D : D being the column with all the email addresses already in HubSpot. Copy paste this formula for the entire row of CRM emails, be careful to leave your computer to work through this if the list is particularly large as clicking anything etc can affect excel as it processes and give incorrect results - always worth spot checking a few results to make sure it has worked correctly.
With the email address being HubSpot’s key contact identifier, you can then filter on the ‘Yes’ results and delete them all (use Find & Select, go to **bleep**…, visible cells only) - leaving you with a file of the net new records only for import.
Be wary of data quality if the file is particularly old as you will get a lot of bounces, may be worth running through an email verification tool first if large portions of the data have been untouched in 3-6+ months.
Hope that helps!