Bill was inflated by adding GST RATE twice

Sir, In the last month my friend has charged the tax for two times...
Actual Bill to be raised was 280/- ie taxable value 218.7/- Rate 28%, but unknowingly he charged the tax on 280/- which was Actual invoice value that was overvalued to 358/- after adding tax @ 28% , the same has done to 20 bills I filed the return for the last month.. but the above explained situation has known to me for this current month. the input was utilised for these wrong bills.. How to account this in GST.??.. kindly response me ..