Doctor in the House S08E01 - When Did You Last See Your Father?

When Duncan finds a baby on his door step with a note attached saying Baby Duncan, he assumes that the child is his and makes efforts to track down the putative mother from his ex-girlfriends. He takes easily to paternity, as, rather oddly, does Dick, and he is dismayed when the baby's actual mother turns up to claim the ownership of her small charge and finds he has no connection with it.