Let's say a Doom engine variant uses the original source code release, and the author decides to stick with the old license because it gives the author more control over the source code (you're not obligated to give it out). Then some code from the Heretic/Hexen engines is added. Already, this piece of software contains code with two conflicting licenses. Then imagine that this hypothetical engine takes some code from, say, the QuakeWorld sources (which were GPL from the start, bless Carmack). Now the engine has code from THREE conflicting licenses!
That's 2 licenses more than are kosher, just in case that means nothing to you.