As stated before, one can recieve communion twice in one day, so long as the second time is the context of fully participating in mass.
This rule is in place to protect abuse of the sacrament. Before this occured, places with lots of churches with lots of masses (say, at 10:00 somewhere, at 10:30 somewhere else, an 11:00 somewhere after that, etc.) Had a problem on Sunday mornings where zealous faithful would catch the last 10 minutes of mass to recieve communion, then leave to the next church on their communion list. Then they could get communion 3-5 times. Hey, more sacrament, more grace!
The church, however, recognizes that just as one’s body does not need to eat more then its fill in one sitting, nor does the soul. To commune with Christ in the sacrament is such a powerful, intimate encounter that more then once a day is not needed. Therefore to limit it to TWICE does not lose any grace, but rather promotes a practice which fosters an increased sense of reverence and sacredness for the Eucharist.
Josh