How Bitcoin system prevents the previous owner of a Bitcoin to spend it? I am a Bitcoin noob.

As per my knowledge, a Bitcoin is a digital signature. 
Owner of a bitcoin knows the private key of it.

If I spend my bitcoin, I still know the private key of it, using which I can spend it later despite not owning it.

Surely there must be something in the system which prevents this. One idea which comes to my mind is modifying the Bitcoin's Digital signature in a way so that the only new owner has the new private key.

I might be stupid and my above query may be just BS. Please guide me in the right direction.
If I spend my bitcoin, I still know the private key of it, using which I can spend it later despite not owning it.


Your are not really "the owner of the bitcoin". It's decentralized. You are the owner of a private key and of an adress.

And you can use the private key to send money away from your adress. Let's assume, you have only 1 BTC on your adress and send it all to your friend. After that, you can't send BTC from your adress anymore because there is nothing on the adress anymore. Only if someone sends BTC to your adress again, you can spend them again. You can't spend the 1 BTC again because it's on your friend's adress. You don't have the private key for his adress, but he has. So he can spend the BTC.