If you sub a player, can that player be subbed in later?
Nope, he’s done for the game. I’ll try to find a cite for you.
Here’s FIFA’s official rulebook [Warning-PDF file]
It’s on page 17. Basically it states that a player who has been replaced can take no further part in the match.
On the other hand, a player that’s been subbed in can himself be subbed out for someone else.
How do I know? Well, it just happened in the England/Portugal match.
Yes…that was a bit silly, wasn’t it?
Presumably it was so Carragher could take one of the penalties.