Does leech stop double power actions as well?

In short yes. A double Power Action in the simplest form is a Power Action that you receive two action tokens for.

There is more to it then that, but your basic question is yes a Double Power Action is still a Power Action.

Edit - Looks like I'm wrong here. It also looks like Leech will stop Epic Actions as well.

Leech will stop double power actions

That is correct. Epic Actions are still Power Actions, they just have other stipulations/rules attached to them.