Can not get skulls Achievements(already have them)

I have all the skulls already in halo 2 anniversary edition but some achievements to get the skulls I can not complete anymore because I have them already, is it still possible to unlock those achievements or not anymore?

Skulls are all unlocked from the get-go for every game. So, collecting them in the campaign is purely for achievements. So to answer your question, you can go back to the relevant levels and collect the skulls to unlock their respective achievements. I think that’s what you were asking, but please correct me if I misunderstood.

In MCC, all skulls are unlocked to the player by default similar to what we have seen in Halo 3: ODST, Halo Reach and Halo 4. However, there do still exist skulls scattered around the maps that should be viewed as collectibles.

The only thing that will happen if you pick up the skulls is that you will obtain the corresponding achievement to the skull if a such achievement exist for that skull. It’s similar to the achievement for collecting terminals (Halo CE, Halo 2, Halo 3, Halo 4), toys (Halo 2), data pads (Halo Reach), audio logs (Halo 3: ODST) and RvB Eastern Eggs (Halo 4 Sp Ops).