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.