How to avoid replicating users when importing a CSV file?

Hello,

I'd like to send an email to 991 users and 661 of those are already registered in our Intercom. The missing 330 seems to have declined the cookies hence don't appear on our system but we have their email addresses.

If I import those 991 users to Intercom, 661 will replicate. So is there a way to avoid this and reach those 991 only once?

Thanks a lot in advance!