The flag will always be listed on the “Old Flagged Posts” page.
No. You’ll only be able to flag a post twice, once as off-topic/ inappropriate/ spam, and once for moderator attention. But you’ll usually only flag a post once (I only realised it was possible when looking into it for this)
Posts you have flagged will display a “you flagged this as inappropriate/ off-topic/ spam / for moderators” below them, but you don’t have any page to list them all unless you’re a moderator/ admin. You can’t see if your own posts have been flagged unless you’re a mod/ admin.
If you’re looking for information on flags, What are Flags and how do they work? is a good place to start. Discourse Moderation Guide also touches on flags, from a moderator POV, as does https://meta.discourse.org/t/draft-flagging-queue-general/58870