Let's face it though, unless the DM is very prepped for that encounter, that brings the game to a screeching halt.
I don't know that the DM has to be any more prepared to run the duplicates than he has to be prepared to run the phane itself (or any of the other creatures in the encounter for that matter). The example you give seems to exaggerate the DM's ignorance, and nothing can really save a DM who can't read a character sheet, or, alternately, can't just tell a player to "use your biggest damage effect against yourself."
A DM who doesn't know what the characters are capable of in at least a general sense, or a DM who can't read a character sheet, has...other problems. Problems that will come up in more than just a battle against a monster.
I could just as easily put up some hypothetical DM who doesn't understand what "weakened" or "insubstantial" means. Really, it's no more difficult (and is, in some ways, easier, since the DM absolutely does not have to have encyclopedic knowledge of character abilities, especially since 4e makes the abilities themselves easier to use and remember).
I mean, just a vague memory of what they did in the last combat is enough to run duplicates: "That ability you used to bump that goblin? It's being used on you at -2."
Alternately, from my own experience running "evil twin" combats, I'm a very improv-heavy DM who doesn't enjoy a lot of prep work, and not being "prepared" absolutely did not in any way negatively effect the combat, because the PC's have done all the prep for me. And this was in 3e, with all of its' fiddly wizard powers and whatnot.
If a DM really needs to prep by writing down a PC's "whatnot" (and I don't really know why he would, since it's going to be WRITTEN ON THE CHARACTER SHEETS, just the same way that the phane's is WRITTEN IN THE MONSTER MANUAL), a moment or two before the game starts, when the DM "looks over" the sheets? Really, is that a tremendous problem? You *chose* to run the phane, you knew the phane involves the "evil twin" schtick, and it catches you by surprise?
I'm getting a little tired of these absurd examples of DM ignorance and awkwardness. A DM that out of sorts will have trouble running goblins ("er...what's the crit value on a dagger? What's the rules for a charge? I'm sorry, I need a few minutes to write down all the stats over again, hold on..."), so it's useless to discredit any specific encounter.
One of the primary goals of 4e seems to be to stop this sort of thing. That creatures shouldn't have abilities that cause the game to come to a screaming halt.
A template that consists of (a) all rolls are at -2, (b) no dailies, and (c) recharged per-encounters is not going to bring the game to a screaming halt. The vampire and the lich that we've seen are more complex than that.
If you want to throw mirror doubles at the party, fine, I'm sure you can do so. But, to put this ability in the standard creature is too hard to use. Never mind WAY too swingy since PC's are supposed to be a fair bit stronger than creatures on a 1:1 basis.
Again, this is a false parallel. The ability CAN BE simple, and there's nothing inherent in PCs that makes this or anything like it "swingy" that can't be solved with a Step 4 for the template. It's no more complex (and, in fact, might be less so) than making a vampire orc.
Not to mention that even if the designers were absolutely terrified of some mythologically obtuse boogeyman DM possibly maybe having to consult a character sheet for his monster stats, so much so that they absolutely did not want any sort of "evil twin" mechanic to be viable in 4e, which is to be some sort of divine sanctum for idiot-proof simplicity, it doesn't negate any of the
other evocative abilities that the phane is loosing.
So, no, that's not really a defense of our rather bland little buddy, here.
Try again without conjuring up phantoms of DMs who are somehow confused into uselessness upon seeing a PC's evil twin.
) Lydia, these spells you have, I don't know this one. What does it do again? What book's that from? Oh yeah right. 
