Sorry, I should have been clearer. Gmail will place messages for user@gmail.com and user+foo@gmail.com in the same mailbox. The grandparent comment talks about normalizing the address by removing stuff after the +. This sort of deduplicates the addresses. Other platforms may have distinct mailboxes for user and user+foo, so you can't strip it on those platforms. The mapping of user+foo to user is non-standard.

There won't be a general approach to deduplicating addresses that map to the same mailbox as the mapping rules aren't always public. But for Gmail, the rule is public, so a best effort deduplication could strip the +.