TMs will only give new moves only...
This was posted by Niantic George...(Niantic Support person on SR). This was the best way to handle IMO...
https://www.reddit.com/r/pokemongo/comments/6i7zwm/whats_changing_in_pok%C3%A9mon_go/dj48y7x/
NianticGeorge Niantic(Verified)[S] 85 points 5 hours ago*
"Hang on. Let me test this. Update: it's random so you take a risk by using another TM.
Update 2: I misspoke. Upon some further testing, I always get a new move when using a TM."
As for "what" you can learn, I'm not sure if say, anyone can learn Confusion or Solar Beam or Dragon Tail/Outrage...
Answers
pretty sure the moves will reroll from that specific pokemons move pool. Almost like when evolving and getting a moveset...you just roll the move again as you did when you evolved it.
In otherwords, I think you will guarantee to get the other quick move that you want as there are only 2 in total, and your pokemon already has one, so you will get the other one that you want instead presumably.
and i guess a 1 in 2 chance to get the exact charge move you want, considering there are 3 and your pokemon already has the one that you currently want to roll.
If so, everyone gets the same movesets. The difference is the IV. I seriously thinks it sucks.
Everyone will have multiples DT/O dragonites, B/SE/C tyranitars, BlisseyZH/DG etc etc
What's the use of other low tier mons then? Everyone will just battle with the best or desirable attackers.
It doesn't make sense niantic
I think you must set a limit