Can you clarify what you mean?  How are you finding out about this?

Are you saying you are trying to prepare 2019 returns and something was already submitted?  Or are you preparing 2020 return (and how is 2019 involved with that)?  Are the clients getting notices after you filed a proper 2019 tax return?  Something else?

The more information you can provide, the more likely we may be able to figure it out.