I had a similar problem a while back, although strictly speaking it wasn't really a "problem" per se. That is, I never had duplicate files on my system and any edits were correctly applied to images even if there appeared to be duplicate folders.
So what did I do about it? To be honest, I cannot recall that I did anything specific. I do remember removing folders and synchronizing and even changing the name on the hard drive to match the case of what Lightroom was showing. My issue seemed to affect only one master folder, perhaps because it was the most used folder by far.
It may have been related to system backups running at the same time. I really cannot say for sure.
Backup the catalog now.
As for splitting the catalog. I don't see that as a necessary step at this point. I would remove the folder and then synchronize a parent folder.
I may also try creating a new catalog and importing from the current catalog to see if the problem still exists. When importing from the catalog you don't need to import all the assets 70k+ at once.
I am referencing LR to a Master hard drive
Mac or Windows? If Mac, is the hard drive containing your photos a case-sensitive volume? Adobe officially doesn't support LR installed on a case-sensitive volume but says nothing about whether case-sensitive volumes are supported for storing photos. However, there have been occasional reports of problems with photos located on case-sensitive drives, and as a plugin developer, I know that LR internally is inconsistent in how it handles the case of file paths. You can find out if your volume is case-sensitive by right-clicking it in Finder and doing Get Info; the Format line will say "case-sensitive" if it is:
I've seen other threads discussing solutions for cleaning up folders that differ in name by case. You might do a thorough search in this forum and the feedback forum. A quick search reveals this thread: Lightroom creates duplicate folder after PS edit or Import from disk,but I'm sure there are others. I have a vague memory that an Adobe employee provided a solution in one of the threads.