Why is this happening???
How Do I fix It?
What were you doing before this happened? Can you reproduce this behavior in a smaller test case?
What OS are you using and as @Hawk said, could you share with us what you did prior to the error?
This seems to be also reported here: Sync Exsiting folders and files now "FOLDER (2)"
My impression is that this happens only(?) In shared folders. A very annoying bug that makes working with code a nightmare (files change name and suddenly code does not work,etc.). BTW, I use Ubuntu (diff versions across diff computers).
The linked post is quite old, and before 1.4.x was released. Insync has improved quite a bit since then, including anti-duplicate capability. It’d be helpful if OP could provide some up-to-date info.