If I enter an already used w2 can you alert me of the error. I can't remember if I filled it last year it not. Please help

Not sure what you mean ... you can only put 2018 income on a 2018 return, 2017 on a 2017 return, etc.

Did you file a 2017 return?  If you didn’t include a W2 on a prior year return you will need to amend that year to add it.  Better print out your prior return and see if it’s included on the Wages line.

No.  The software will not know if you already used a W-2.  TurboTax is software for a do-it-yourself tax return.  Look carefully at your W-2.  It has the year on it.   You should not be entering a W-2 from any other year in to your 2018 tax return, if that is what you are working on.  And employers have not even issued 2018 W-2's yet--they are issued in January.  Please explain what you are trying to work on.