Even with prefetch enabled having 2 CopyOnRead directories pointing to same index can lead to one of the instance reading index files directly off of remote index.
The reason this happens is because COR#copyFilesToLocal explicitly chooses to work with remote if index copier reports that a copy is in progress.
This wasn't a problem earlier when COR was only used via IndexTracker so concurrent COR instances weren't expected (COR's avoid local for conc copy was probably worried about non-prefetch case).
OAK-8097, DefaultDirectoryFactory also uses COR to bring the files. Which means that if there's a query against an index which is getting updated as well then either of COR instance could read directly from remote.
The condition should only be relevant during early app start up but since this can happen in default configuration, we should attempt to fix this.