It's been pretty established by the Wolverine movies that movie Adamantium can cut movie Adamantium. It's not like in the comics.
1- Adamantium bullets can penetrate Logan's adamantium skull.
2- Adamantium blades (Deadpool) could have decapitated Logan, as "hinted" on screen.
3- Adamantium katanas cut Logan's Adamantium claws.
That still doesn't explain why Logan's claws don't cut each other in the mirror scene (Origins), but I assume Adamantium can cut Adamantium only when it is given the proper speed and momentum.