Although the above seems much more probable, I thought I would bring to your attention an alternative suggestion that also revolves around illegitimacy.
The IGI states that a Samuel Douch married Hannah Harris in Ninfield on the 27th December 1794.
Their son 'Samuel Douch' was baptised in Ninfield on 14 June 1795, which indicates Samuel Douch was conceived out of wedlock, and to the more pedantic ecclesiastics of those times, even though his parents were now married, he was still only entitled to his mother's maiden surname rather than that of his mother's new husband and his presumed father. Therefore Samuel Douch born Ninfield 1795, could have also been known to church authorities as Samuel Harris. I feel it may be worth considering this as another possibility.
A sister Mary does not seem to fit comfortably into this scenario, but wonder if the William Harris who married Jane Fuller was related to Hannah in some way?
Roy G