If I use red eyes dmd effect to bring a dragon from the graveyard then send red eyes dmd to the graveyard and then bring it back in the same turn, is it treated as a new monster resetting it so I can re-use its effect to special summon another monster?
Yes, you can do that.
This comes down to the same rulings involving this question: Tri-Gate effect
Basically, the rule with once per turn effects is as follows:
If the cards simply says, "once per turn you can..." or any variation on this. It basically means once per turn, per card. If the monster is removed from the field and summoned again, this state resets because it no longer counts as the same card.
On the other hand if the cards states something along the lines of "you can only use this effect of "Monster Name" once per turn", then the effect can only be used once per turn for any card with that name. That includes any other physical copies of that monster you would summon.