Descent rulebook page 16 paragraph 3 - Each card specifies when it may be played. Two Overlord cards with the same name cannot be played on the same target in response to the same triggering condition. After resolving the effects of an Overlord card, place it faceup in the discard pile.

So using Dash or Frenzy on the same monster twice is a no no...correct?

That is correct.

but you could play them on 2 different monsters right?

also correct. Two cards under the same name can be played on two different monsters of the same name. They are two different monsters.