If the Hugh that you're trying to delete has other 'branches' of the tree coming of him then the tree won't let you delete him because that would split the family. Try deleting the other one instead. If they both have branches from them (I presume they are identical branches), then you'll need to delete one or both of these branches, which could take a while.
Try changing the details of one of the Hughs. Then try deleting him.
If this doesn't work, then it might be a bug. You can always write 'Ignore this box' in the name section as a stopgap solution, but I'd advise reporting this as a bug.