Is it possible to select the same character twice?
In the arcade version this is possible via a glitch, although it's extremely difficult to do. Select your first character, then move the select cursor to the far left or far right of the character select grid. Hold left or right and the random select will activate. If you can time it correctly you can select your first character again. I have never been able to do this myself in the arcade version and it's possible it doesn't work in all versions of it. In the SEGA Saturn version there is a cheat that lets you pick the same character twice, see the secrets page for more info.
What are infinite combos?
Combos that that never end and the opponent can not get out of no matter what. Many fighting games have these, and that includes Children of the Atom and Marvel Super Heroes. However, X-Men Vs. Street Fighter really put these into the spotlight. The easiest one in the game can be done with Ryu and Akuma. Use their hop kick (forward + forward kick) then follow it up with a low strong punch, then keep repeating this. This infinite combo only works in the first version of the game, but it just shows how easy they could be.
How did this idea come about? Why X-Men versus Street Fighter?
For Capcom it was something they could do with the Marvel licence, I guess. It would have been relatively quick for them to make, also -all of the Street Fighter characters aside from Cammy are using their Street Fighter Alpha 2 sprites, just with a few new animations here and there. Likewise, most of the X-Men characters are from X-Men Children of the Atom. Only Rogue, Gambit, Sabretooth and Apocalypse are truly new.
Marvel most likely approved it because they were notorious for approving everything at the time. The company was facing bankruptcy in the mid-90's, and was well aware of it, so trying to get things with their name on it to sell was top priority. If you think the concept of Marvel characters fighting Street Fighter characters is ridiculous, why not check out X-Men: The Ravages of Apocalypse -a Quake total conversion which has you playing as a cyborg Magneto built, which has to fight robot clones of the X-Men.
Are there different versions of the game like there were with Children of the Atom?
There are apparently three different versions of the game. Version 1 may have only been released in Japan and contains the most infinite combos, including the Ryu/Akuma hop kick one. Version 2 was the main version of the game that removed some infinite combos from the first version. Version 3 is the one that fixed up a lot of the infinite combos and was the version the SEGA Saturn port was based on. This version may also have been Japanese exclusive. Unfortunately the title screen doesn't give out a version number like Children of the Atom's did, so it's difficult to tell which version you are playing without testing out if certain infinite combos work or not.
So how does this game fit in with the Street Fighter canon?
This game is not part of the main Street Fighter canon. Instead it takes place in it's own separate universe (the Marvel Earth number for this game and the other Capcom fighters is Earth-96169). It seems Capcom wanted to try and make some story progression with the Vs. games, as seen with what happens to Charlie in his ending, and then Shadow's appearance in Marvel Super Heroes Vs. Street Fighter.
That said not much plot has ever been given out for this game. The manual for the Playstation version of X-Men Vs. Street Fighter states that many street fighters and mutants have gone missing and Magneto may be behind it, so teams have gotten together to get to the bottom of it.
What's with the translation?
The English translations for the previous two games were good, but this one isn't quite up to standard. Some of the win quotes come across weirdly and some are hilariously stupid (such as the one above). Check out the game's bizarre description of how to do aerial combos -eh? Get into the striking distance at once! Gee, great tip Capcom, thanks ever so much. All is Confusion! Bright lady, victory! An honorable fighter with great honor! This game always seemed a little rushed so it doesn't surprise me that not as much time was taken with the translation as there should have been.
Who are all the captured mutants and street fighters in Apocalypse's stage?
Out of the ones you can clearly see, the captured street fighters are Dan, Sakura and Blanka. I can't name all of the captured mutants but the ones I do know are Beast, Forge and Psylocke. Shuma-Gorath can also be seen next to Blanka. It is possible to break some of the capsules and release the captured characters by jumping at them, or taking a hit when next to them.
Weren't there some rumors about more characters being hidden in the game?
There were apparently some rumors flying around thanks to certain American games magazines. Some players believed that there was a way of fighting or playing as the smaller version of Apocalypse, and that Doctor Doom and Thanos were hidden characters. However, none of these things were true.
Any voice actor trivia?
Most of the new X-Men characters in the game were voiced by the same people who voiced them in the cartoon. Lenore Zann voices Rogue, Don Francks voices Sabretooth and Tony Daniels voices Gambit (Tony was the second voice actor used for Gambit in the cartoon).
However Capcom didn't manage to get John Colicos (Apocalypse's voice actor in the cartoon), so they got Lorne Kennedy to do it instead. Lorne also does the voice for Magneto in the game. Most of the Street Fighter characters are using the same voice actors from Street Fighter Alpha 2. Cammy wasn't in Alpha 2 however, so a new voice actor was used: Susan Hart. Susan has worked as a voice actor director/producer for several other Capcom games, including Dino Crisis and some of the Resident Evil sequels.