Its pretty simple. I'm creating artboards. Every now and then a number gets duplicated. At first I was drag/copying the previous artboard so I also tried just drawing a new artboard but it still duplicated the number. This has been happening for years. Currently I'm using the latest version of Illustrator so it's affected pretty much every version over the previous couple of years. The problem with this duplicate number is that if I decide to export artboards, one of the duplicates will not get exported. That's how I discovered this problem.
can you share a screenshot of your "artboards" panel in "list view"? are there genuinely 137+ artboards in that document? Or has there been many artboards created and deleted and the index just keeps rising?
if you open a brand new document and create 100 artboards... do any of them have identical indeces like in your screenshot here? Or does the problem present itself on files that you've opened but were created by someone else? What happens if you delete one of the duplicate artboards and then create it again. does it still get the duplicate index?
The more info you can give us, the better we'll be able to help.
Yes, there are 137 artboards. If I delete, then create again either by drag/copy, or just drawing it, it duplicates that number again. The original documents are created by me, but I'm revisiting these documents and adding/removing. Today I came back to this document and added the artboard and it made the duplicate number, then I noticed the earlier duplication there as well. That might be a clue as you're suggesting.