Wellington in Somerset has problems with receiving mail for Wellington in Shropshire, as well as receiving mail for other places. Maybe as a result Wellington has had a number of cachets for use in those kinds of situations.
These first three sheets have examples of different "NOT KNOWN / WELLINGTON SOMERSET" cachets, a smaller one from 1965, and a larger one from 1979 (two) and 1986.
A slightly different example is this framed "MISSENT TO / WELLINGTON / SOMERSET" cachet, which was deployed on the example below because it had got missorted to Wellington rather than Wells (Wells, Somerset is another place which potentially gets mail for another Wells in Norfolk though that has now been renamed to Wells-next-the-Sea).