How are addresses kept unique?

[hjordan]

How does bitcoinaddress.org generate unique addresses when it's used offline because it can't be checking a database of addresses to check it's not generating a duplicate address?

[Colin Miner]

How does bitcoinaddress.org generate unique addresses when it's used offline because it can't be checking a database of addresses to check it's not generating a duplicate address?

I don't think offline/online has anything to do with it.

All addresses are created in a pseudorandom manner and don't check that they already exist.
Its just very unlikely two address will be generated that are the same.

[tunctioncloud]

How does bitcoinaddress.org generate unique addresses when it's used offline because it can't be checking a database of addresses to check it's not generating a duplicate address?

Generating duplicate address is unlikely, unless the random function is flawed
If you could generate already existing address in blockchain with Bitcoins siting there, you could spend the Bitcoins