The next number is 109 and we are both correct except that we are briefly isolated by our Local Files. No isolation after the SWC and the error(s) is/are trapped. The central file does not oversee the delivery of unique Mark values, maybe it should? It only reacts to work done in our local files.
Some of these errors, like our doors, get resolved as the design progresses. A door number gets a better value as soon as room numbering is sorted out because we endeavor to define the door number then. Others don't get resolved naturally because we don't usually itemize them, but Revit does.
An example of this is Air Terminals in Revit MEP. They get tagged but not usually by a unique mark value like a door. We could but it doesn't happen in practice very often. Instead they are usually tagged by Type, like windows are often tagged by architects. Speaking of windows, they suffer from this issue too. The same scenario exists for these yet there is no natural process to resolve the mark values other than to spend time fixing them.
In this instance, and other similar items, Revit creates extra work for us because it is necessary to minimize warnings in order to ensure the best performance while working on our projects. It might be worthwhile to have a way to define which elements Revit should incrementally assign a Mark Value. For example, hypothetically, we could tell Revit to ignore Air Terminals via Mechanical Settings. This would eliminate a lot of warnings when several people are building their model simultaneously.
In the meantime we need a hero API programmer to provide a free application (or reasonably inexpensive) to renumber all items of a user selected category so they all have unique Mark values again. Since we don't care what their real Mark value is the application can be merciless and simply renumber them all starting with 1.
[Added because of a comment, thanks!]
I dismissed the Element Positioning tool in the past because it didn't seem to work properly for me, I've revised my opinion today, user clumsiness. This tool is part of the Extensions for Revit, available via Autodesk Subscription only. There are versions for each Revit flavor. Revit Structure (RST) has the fullest collection of tools because they were originally written for RST and later the ones that would be useful for RAC and RME were made available. I decided to make another VIDEO to explain how it works. You can listen and watch here too.