I've been thinking about this for awhile, doing a photo study of how people in twins maintain/renovate their homes. Living in a twin myself, and my neighbor at this moment installing a new front door, this is something I have some ideas on.
Dormers on twins are a place where people might cut corners. Like this one. the house on the left has the original ornate setup, the middle window flanked by very narrow sidelights. Stylish, but to replace today would most likely be quite pricey.
The home on the left didn't worry about that; when it got sided, the whole thing was just replaced with a stock window. On its own, it might not be noticed, but in cotrast to its mirror twin next door, well, its like gravy and ice cream.