State tax filing

I am being asked to repay (with interest of course) NYS for the $5,000 tax credit I received on my 2018 tax return. The reason stated is I can only receive the credit once. I claimed the credit in 2017 for a system installed and claimed in 2018 for a SEPARATE system. The attorney for NYS is saying the max claim is $5,000 however the letter from NYS tax and finance states "the maximum solar energy system equipment credit PER ELIGIBLE SYSTEM is $5,000.00" DO I have a case here to fight this?