Give all pre-existing duchies, i.e. one where the duchy name matches the city name, a one-shot name change? I'm sure you'd get a lot of takers, given the opportunity. Should be possible to filter it via code, so that ineligible requests aren't even accepted. Drop the "duchy of" string, and compare the results with the list of regions in the duchy. If you get a match, show the "Change Duchy Name" option. Leave it active for a week, then yank it out.
Or we could use something similar to name requests, and have the Duchy Name changes recorded pending approval.
