What do you call a person who doesn't get offended? [duplicate]

Thick-skinned is a possibility:

thick-skinned - adjective
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Someone who is thick-skinned does not appear to be easily hurt by criticism:

• You do need to be thick-skinned to survive as a politician here.

Cambridge Dictionary

imperturbable: incapable of being upset or agitated; not easily excited; calm: imperturbable composure.

​Unflappable is an adjective you may use:

• not likely to get worried, nervous, or angry even in difficult situations:
  • She's totally unflappable - you have to be when working in such a high-pressure environment.

(Cambridge Dictionary)