Crysis 2

This is achieved in multiplayer. When you kill enemies, they drop a dog tag. It is indicated by a tiny beam of light pointing upwards. All you have to do is to walk over the corpse to collect it. Dog tags don't have to be unique, any 20 will do.

This solution is only for those people who have had this glitch out on them and not unlock after picking up 20 dog tags in mutliplayer. I take no credit for this solution, as it was suggested to me by somebody else, and well documented to work for certain other games. Similar examples can be seen here:

Solution for Can it really be that simple? In Army of TWO: The 40th Day
and
Solution for Mister NO it all In Command & Conquer 3: Tiberium Wars

PLEASE NOTE: This will reset all of your multiplayer statistics. So if you are still working towards other multiplayer achievements, finish those first!

The exact steps that I followed were:
1. From the dashboard, move your profile onto a USB memory stick
2. Load the game. As soon as the EA logo comes onscreen, remove the USB stick. This will prompt you to reinsert the stick, but don't do this yet.
3. Press Start, and then choose multiplayer. From here go to Operative Status then Service Record. Go over to the General tab and you should see all of your stats reset, including "Dog tags collected" at 0.
4. At this point, put the memory stick back in the xbox.
5. I then played 4 consecutive games to collect 20 dog tags, checking that the count was going up correctly after every match. The achievement popped exactly when I picked up my 20th dog tag in game.

I hope this helps those of you who have had this glitch on them. Before this, I had collected over 400 dog tags without the achievement popping.

Please let me know if there are any questions or clarifications needed regarding any of this. Good luck everyone!