Is not it possible that a same address is being generated by 2 clients?
As long as the wallet is using a source of random numbers with significant entropy? No.
If there is a bug in the computer such that the "random numbers" that the computer provides aren't "random enough", then yes, absolutely.
Or 2 persons at 2 different parts of the world are creating brainwallet from same passphrase ?
This is VERY LIKELY.
This is why people should not use brainwallets. Most humans aren't capable of creating and memorizing a brainwallet passphrase that is random enough to reduce collision chances to acceptable levels.
