Yeah, or two Stackers. When you trigger the rockslide area, you'll randomly get a Stacker or Johnson. So if you've already got one Johnson (namely from the early part of the level), or if you've already got one Stacker (namely from the hillside survivor area), you can end up with two.

That two-Johnsons picture PerseusSpartacus showed from my site is here by the way (where I talk about this stuff), and the shot with four copies of the same Marine appearance is here (they had different voices though).

: What about
: a Scorpion with four sarges on Assault on the Control Room?

In AOTCR you can get two Johnsons, but that's the max. The initial Pelican contains Stacker or Johnson at random, and by doing the seven extra Marines trick you can stop the game from killing him off, which would normally happen. Then there's the sniper Johnson in the crashed Pelican area of course.

Johnson can appear four times in POA by the way, but only two of those occurrences are in the same area.