This takes place on the first mission of doom dark ages. Doomslayer just got dropped from the maykr ship and deployed onto the sentinel village. You are a sentinel fighting against the demons from hell.

So Doomguy woke up to see a exact clone of his looking at him in the corner. The fake Doomguy, (aka the fake Flynn taggart) is a dangerous shape shifting creature. Samuel now has to kill the fake one.