Ho and just one silly question, when a new address is generated, how to be sure it is unique ? Is it time based ? And what happens if 2 addresses are generated at the exact same time ?
It depends on how you generate the bitcoin address. If you are using the current version of Bitcoin-Qt, then the address is generated from a random number. Because the size of the number pool is so large, the odds of generating the same random number as someone else is astronomically small. So small that it is for all practical purposes impossible.