Does the protocol ensures that private keys are unique? There is a very very low chance for the same private key to be generated. (source)

However, what happens when my generated private key matches a private key that is already used?

Does the protocol reject private keys if they are already used?

If not, what kind of damage will be done?
However, what happens when my generated private key matches a private key that is already used?


Then you got lucky, and can spend someone else's coins.


  Does the protocol reject private keys if they are already used?


No, how should it? Keys can be used more than once in normal operation.

Fortunately, it is extremely unlikely that this ever happens. See also these two related questions.