This usually happens because you are renumbering sheets with numbers that already exist in your document.
Another cause is this problem: 1. Renumber set of sheets A, 2. One of the new numbers exist in the set A, but it doesnt matter since you are renumbering all of them anyway, 3. Process in Dynamo begins, but renumbers a sheet while one of the other sheets in Set A has the same number (i.e. this sheet needs to be renumbered first to ‘free-up’ the number) otherwise the process fails and you get the error you report.
I’ve written a package which includes a node to renumber/rename sheets which is designed to circumvent this problems and report where it finds duplicate names: bimorphNodes bimorph.RenumberOrRenameSheets. Give that a shot and see if you have any more luck.
