The level begins with just the Marine and his clone, who is an ally for mere seconds, until he turns against the Marine and tries to kill him. After defeating the clone, more hostile copies of the Marine appear, each armed with a Ray Gun C-01y. After battling and taking out each clone, the Marine survives and completes the level, concluding Level 22.
These are visual replicas of the Marine, with a total of five in this level. Each are armed with a fully upgraded Ray Gun C-01y, except for the first clone, which has the same weapons as the Marine.
Because of the fact that the first clone has all of your equipment, you can use a large amount of Grenades without having to worry about running out.
It is recommended to switch to the Lite Battlesuit mode for this level, as it will make your clones a lot easier to kill. It doesn't affect you as much, since you have Time Warp and can deflect Ray Gun shots.
If you use PSI blades right after your clone turns on you, you can kill him without getting harmed. This works with next 2 clones as well. However, keep in mind that the clones may counter-attack.
The Marine's clones are the only enemies in the level.
As Eric Gurt has revealed, the clones are Civil Security soldiers inside copies of the Marine's suit. However, it's not clear how could Civil Security do that.