Doug's friends were originally supposed to be a one time appearance.
If the Heffernans kitchen and it's blue refrigerator look familiar to TV viewers it's because it was originally used on the ABC sitcom "Grace Under Fire." (1993-1998) which had been axed by that network 7 months earlier due to difficulties with star Brett Butler.
In Season 1 and 2 "Sarah" (Lisa Riefell), is introduced as Carrie's sister early in Season 1. Also, "Richie" (Larry Romano) who is a Bronx firefighter, is Doug's childhood friend. Interestingly, both characters disappeared, "Sarah" was gone after only 4 episodes and Richie was in the show for a total of 44 episodes. There was never any explanation as to what happened to the characters, in particular, Carrie's sister, who just "disappeared' after only a short run on the show.
The line where Arthur gives the priest money and tells him to get an egg cream originally had Arthur telling him to "go get yourself an altar boy." But CBS refused to let the line air and it was changed.