Well guys there's some fresh info on this.
This customers cheque was dated 3/11, this was the day I last cleaned his windows.
I now notice that the cleaned your windows slip he returned to me was dated 3/10.
He posted that cheque the same day I cleaned his windows, but before I had done them, he was paying for both properties, but for the previous clean. If he had paid a bit earlier I would have known to leave the other house.
I am glad I slept on it now, it wasn't as bad as it looked. He may still come good.