We forget that once God forgives a sin it is forgiven and He remembers it no more. We should not be looking for a second forgiveness but remember God says "I will remember it no more".

If we ask God to forgive that sin again, there is nothing in the scriptures to say He will do it. If God did have an answer it would probably be something like "What Sin"? That is, for what sin is it you are asking forgiveness?

While the term "What Sin" is not a bible term it does give us something to think about knowing God has already forgiven that sin. I heard this from a sermon many years ago and have remembered whenever my forgiven sin comes back to me; don't forget, ME, God has forgiven that sin.

As always, there is someone who will say;
"If God knows everything how can He remember a sin no more"?

There could be a multitude of answers to this question but none of them should change what the scriptures say.

The best answer to this is do not try to 'explain' what the scriptures mean. Because the bible says what it means and means what it says there is no need to try to 'explain' what the scriptures say.

There is more information about 'explaining' the scriptures   @   The Bible is Simple to Understand