I have different question besides coding. It's about API so I though it's not that bad idea to ask it here.
Blockchain has API like that: https://blockchain.info/api/api_receive but every time user wants to make a transaction it will generate a new address for him, but my question is. Could I use same address over and over again? Ex: I'll generate a address for user once he registers. If he sends payment there, callback url will still be called even if he has done it multiple times.