Union: for every single transfer rclone listing all the remotes

[onedriveunion]
type = union
upstreams = [20 onedrive business here...]
cache_time = 604800

Command:

rclone sync onedriveunion: /path/to/store --check-first --transfers=8 -P

Issue: The problem is that each time a file needs to transfer, Rclone checking all the remotes for the file, until the file is found. So, in every single transfer rclone listing all the remotes again and again. It's not remembering the location even when --check-first is applied.

We need some caching here. Otherwise it's practically impossible to use Union.

Here is what happening:

1) File1 needs to transfer.
2) Searching File1 for all remotes. ------------> This is the problem.
3) File1 found on OneDrive5
3) File1 transfer complete.

1) File2 needs to transfer.
2) Searching File2 for all remotes.  ------------> This is the problem.
3) File2 found on OneDrive8
3) File2 transfer complete.

Start by tracing how Union handles remote listings during rclone sync, particularly with --check-first, and identify where each file lookup repeats the upstream searches. Done means the lookup location or result is cached so repeated transfers do not list every remote again; add or run coverage for a file found on different upstream remotes.