Query regarding release and join a new company

I had received an offer from company A and after that I relieved. When I am serving the notice period, then B company approached me but during interview I did not inform them about notice serve. Now B company selected me and they ask me about resignation copy. Now how do I tell them that I have already relieved and join to your company as if they ask why don't you tell us about relieving earlier?

Hi,

I trust you got relieved properly from Company A. In that case there is no harm in sharing info that already you were serving notice period with Company A, while attending interview Company B. As long as relieving is proper nothing to worry.

B company questioned me that why don't you tell us that you are serving notice period while interview? Now you are telling this matter after receiving of offer.
But my reliving is valid with proper documentation. So what should be my answer in this case?

Hi,

You may provide some fitting answer like " though I submitted resignation my ex-employer was not very keen to relieve me and was forcing me to continue " etc.