Hi @FirstNameL80914 ,
Contacts are determined based on unique email addresses, so if you upload a contact list with different first name, last name, physical address info, etc. alongside the already-existing email addresses, it will simply overwrite the original info.
Any contacts that have previously unsubscribed will not be re-added to any lists. The rest of their info may be updated, but they will still remain unsubbed. As far as other "bad addresses," are you referring to contacts that you've deleted before, but weren't unsubbed (such as bounces or low engagement)? That may cause a complication, which means it may be worth exporting your active contacts, and then updating all the applicable info outside of our system, before performing the re-upload.
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