[Venkman raises his eyebrows.]

You forgot? Damn. Maybe I shouldn't tell you, then.

... It was the uniforms that got covered in bits of Stay Puft and turned into weird copies of us. When we captured them and put them in the containment unit, there was something about the spirit energy residue... I can't remember the exact wording Egon used.

But basically, when the fake versions of us went into the unit, they all meshed together to form a new Stay Puft.