How does bitaddress.org ensure unique address? You can use this service offline (as it's done with JS) so how can they possibly ensure a unique address is generated?
I don't know what bitaddress.org does exactly, but just take some numbers into consideration: 

There are 2^160 possible Bitcoin addresses. If you generate an address from a key pair generated with a good randomness source, it is fairly impossible to find a collision. 

In conclusion, they can ensure uniqueness just by the negligible odds of picking an address more than once.