Hi,
You can use data import wizard and create contacts. If you have defined deduplication rules properly, it w'ont create duplicates nor update your current contacts
If you want to update your current contacts, you will have to use same data import, but in your contact import template, you will define contacts GUID. You can export all you contacts, and on the left columns you will have your GUID.
I won't recommend to give end user access to import feature, too many errors handling. Just give them template to complete.
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