If you activate "The List" then return it back to hand, Replay "The List" and target the same unit. Does the Minus Might Stack

Yes — the -2 [M] stacks. Each activation of The List creates its own "this turn" modifier on the unit, and nothing about The List leaving the board removes the first one.

The List's ability is [T]: Give a unit with the named tag -2 [M] this turn. Once that resolves, the -2 sits on the targeted unit for the rest of the turn regardless of what happens to The List (no "while I'm on the board" clause). Returning it to hand and replaying it makes The List a new object, but the unit is unchanged, so the new activation applies a second, separate modifier — the two are summed in the Might layer for a net -4 [M].

Two practical notes: when you replay it you re-name a tag as it's played, so just name the same tag again, and a non-unit Gear enters ready (Rule 359.2.d), so it can be exhausted for the second activation. This is also the only way to fire it twice in a turn, since the [T] cost exhausts it and gears don't ready on their own.