Considering that you will need to export and update your .csv to identify which records are duplicates before re-importing, there is no 'quick' way to mass merge. Typically, when talking about mass dedup, quick = mass errors! 😐
If you don't want to do it manually using native excel functionality you might want to check out https://dedupe.ly/ who is specializing in finding and merging duplicates in CRMs (including Hubspot of course).
Considering that you will need to export and update your .csv to identify which records are duplicates before re-importing, there is no 'quick' way to mass merge. Typically, when talking about mass dedup, quick = mass errors! 😐