Just keep it running. Its not a big deal unless you are super-duper conscious about it.
Thousands of vehicles are there on road with duplicate number plates and they hardly came into notice unless they really look like fake.
Ask ex-owner if he has the original plates with him or those are lost or what happened with them?
In case if you are ever stopped by excise police and they tell you that your plates are not original, just tell them the same story that I bought the vehicle right in this condition and didn’t know they are duplicate.