In my profile for Robert Trewinnard both his marriage and the baptism of a number of his children are listed in two different parishes in Cornwall.
I think one represents where the marriage/baptism actually took place, and the other parish , Mawnan is where Robert's father Joseph was the parish priest, so he was keeping a record of his family?
The other option is because they were a wealthy family, they might have had property in both parishes and as a courtesy records were kept in both parishes?
Perhaps the same thing is happening in the Doude family in Kent?