Writing to the Chicago Tribune despite the apparent loss of his Caps Lock key, Ramis revealed that at least a couple of the original cast are definitely onboard for the third movie:

yes, columbia is developing a script for GB3 with my year one writing partners, gene stupnitsky and lee eisenberg. judd apatow is co-producing year one and has made several other films for sony, so of course the studio is hoping to tap into some of the same acting talent. aykroyd, ivan reitman and i are consulting at this point, and according to dan, bill murray is willing to be involved on some level. he did record his dialogue for the new ghostbusters video game, as did danny and i, and ernie hudson. the concept is that the old ghostbusters would appear in the film in some mentor capacity. not much else to say at this point. everyone is confident a decent script can be written and i guess we'll take it from there.

The same acting talent as Apatow's other movies? So that would mean Paul Rudd, Jason Siegel and the by-now ubiquitous Seth Rogen for the main roles, I guess... but who will be the new Annie Potts?

Harold Ramis confirms 'Ghostbusters III' [Chicago Tribune]

Keaton ends up making two copies of himself: a hard-assed construction worker version to handle work, and a prissy neat freak version to handle his domestic chores. The only rule, as he explains here, is that neither version can sleep with his wife. Of course, they both end up doing her. And so does the genetically damaged clone of a clone whom they create later. The only funny sequence in the movie is when the wife, played by Andi MacDowell, pours out her heart to the mentally challenged clone of the clone.

Sadly, Multiplicity isn't even the best comedy about cloning featuring Eugene Levy in a supporting role, which ought to be a fairly easy category to rise to the top of. That honor belongs to Repli-Kate, a far superior movie.