THE STORY OF DOOM2:2 After you've destroyed the icon of sin, earth gets re-built, countrys come together, everyone's happy. 5 years after your retirement, you get a call from your old commander, Commander Ragg. He calls you up and goes "TaaaGGGAAARRRTTT!!!! The military has recently built a prototype time machine, and we need some tough marine to test it out. What do you say, buddy?" You think about it for a sec, and answer "...you got yourself a time machine pilot" You enter the time machine, deep in a secret part of a new military installation. The time machine activates and you get thrown 5 years in the past. You emerge from the time ship and look around... oh god- not again! Yep, it looks like you landed at the beginning of the demon invasion of the starbase. Oh well. Since the time machine is broken, you're going to have to fight them all over again, through exactly the same levels.
From scratch apparently
9 months of carefully creating the levels identical to the ones of Doom2