Question: If Someone is already tagged on SteamRep, should I report them?

You can always report them - if they are tagged with multiple offenses we won't be adding more notes to their profile to avoid just continuous stacking of notes/tags so two has been the cut off lately. If a user is reported multiple times its pretty clear they have no intentions on stopping.
Even archived reports from someone that was tagged multiple times in the event they try to appeal will help against that with added history.
Thing is if you accept a friend invite from someone - you should be checking their history be it with google, or steamrep here to see if they have any notable history.
If you see they are tagged I would cut contact at that point if it was me - but if you wish to trade with the user we can't stop you.
We want users to SLOWWWW down and check each other out prior to trading rather than get to the point they were scammed and then making a report.
This make sense?
So if you cut the user loose after being tagged and you finding out then just report them to Valve as they are the only ones that can trade ban someone.

Looking at your post history - this report
https://forums.steamrep.com/threads/report-76561198194367461-tf2-team-fortress-2-items.163704/

Yeah this guy has about as much of a chance at any successful appeal as me getting hit by lightning twice..while winning the lottery and finding the worlds largest diamond all at the same time. So report was noted, archived and we went on to the next one..which of course we would want to concentrate on those reports that of users not already tagged with what free time we have.