Does God condemn a person for rejecting Him ?  Or does an already damned person remain condemned when he rejects God ?

I believe it is the latter.  Unfortunately, the partly inaccurate and truncated message that people often hear is that they will go to hell for rejecting God.

This is what John 3 : 17 - 18 actually says –

For God did not send His Son [ Jesus ] to condemn the world, but to save the world through Him.  Whoever believes in Him [ ie. Jesus ] is not condemned, but whoever does not believe already stands condemned because he has not believed in the name of God’s one and only Son.

Paraphrased –

A drug is invented not to kill a patient, but to cure him.  A patient who takes his medicine gets better, but a patient who refuses to take his medicine is as good as dead, because he has not taken the only thing which can cure his otherwise terminal illness.