* It possible to generate the same address twice

* It is extremely unlikely, you're more likely to be hit by a falling asteroid next time you leave your house, several times in a row

* It would take millenia to generate all addresses if you generate only a thousand per second

* Coins are attached to addresses, not to online people. Two people who have the same address share the bitcoins attached to them, and won't notice until the other one spends some.