Can i work for my ex-H1B Employer w/o H1B Transfer

Suppose I am working with Employer A on H1b. Now employer B is ready to hire me. Suppose I start working with Employer B and my previous Employer A dont withdraw my H1b. Now after a year if i decide to work again with Employer A, do I need to go through H1B Cap Exempt Transfer, or i can directly start working with Employer A, as it already have a H1 for me.