Hi,
Successive data imports have led to many duplicate contacts in HubSpot.
How do I quickly merge the contact data files without losing data stored in the records?
Thanks
Simon
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Q: How do I quickly merge the contact data files without losing data stored in the records??
Short A: see [Now Live] Import, Deduplicate, and Search Objects Using Object ID
Longer A:
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! 
Always happy to help you build on HubSpot.
Best,
Frank
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).