So much for a "couple" of questions eh? :o
Anyway, IF your soldier dies (from your question I'm assuming your just asking about infinity mission) during the Infinity Mission, you CAN get that soldier back. To do this simply play extreme mode (Better chance at finding that character) and check your map during each stage for prisoners and he/she has a chance at being one of those prisoners.
No it is NOT possible (highly doubt it) to get more than one of the same Unique Character.
Yes there is the Kojima soldier, that soldier's camo is Red and Black with Kojima lettering on it. Check Alexander's Signature for a picture of what that soldier looks like.
You can ONLY lose your characters(depending which game type you play) if you play on the RC games (Real Combat). If you play any of these game types yes you have a chance at losing your soldiers. My advice would be if you REALLY wanted to play RC games (for camo farming or for fun) then use soldiers you don't really like or need or are weak. That way you can White Flag and FULTON without any worries of losing soldiers you want to keep. To sum this question up, Yes you will lose that character forever if you lose him in a RC game.
- Hope that answered your questions.
ALSO if you want some more questions answered, check out the FAQ I did here:
http://mgsforums.com/index.php?showtopic=21540