That means someone else has already filed using the same social security number - not you. You might want to ask the client if they have had any issues with ID theft, and look into filing Form 14039 in case that is the situation. It could simply be that someone submitted a tax return done manually and either wrote the wrong ID  number on their return, or the IRS employee  made a key-punch error when entering the tax return. But to be sure, you should pursue the possibility of ID theft too, to get ahead of it if needed.

Often that type of error is also caused when a dependent files his own tax return and claims himself/herself, and then the parents file and try to claim the dependent on their tax return too.  (not as important now with loss of exemptions, but other items such as education credits or HOH filing status might be affected.)