What happens if a shadow move hit a shadow pokemon?

Question

Will it be super effective, not very effective or normal effective?

NOTE: This is referring to Pokemon Colosseum and Pokemon XD, NOT the ghost or dark types in the regular Pokemon games.

Answer 1

According to bulbapedia:
In Colosseum, Shadow moves are regularly effective against every other type.
In XD, they are super-effective against non-Shadow Pokemon (2x) and not very effective (0.5x) against Shadow Pokemon.

Answer 2

It will have a x4 resistance to the attack, result in a Base power of 10-25. This makes for a very weak attack when it's shadow against shadow, which is why Shadow Pokemon slowly learn new moves as they're purified.