Although I think the problems of having two Dukes of Edinburgh would have been insurmountable, I have previously suggested that, had the late Queen not been confident that King Charles would have carried out her wishes (which, in my view, he has, shamefully, failed to do) it was open to her to elevate Prince Edward to be the Duke of Edinburgh-on-Leith (or some such).

The historic, co-extant Dukedoms of Newcastle upon Tyne and Newcastle-under-Lyne provide a precedent.

https://www.nottingham.ac.uk/manuscriptsandspecialcollections/collectionsindepth/family/newcastle/newcastlefamilyhistory.aspx