[P_Jamula]

It's not possible. Kodi does not support a global actor thumbs folder.

There's two issues the OP is addressing - the apparent duplication of local artwork due to the texture cache, and the lack of a global actor/cast folder.

All local artwork will be "duplicated" by the texture cache - that's the point of having the cache, although it's not really duplication as the cached artwork is optimised for the local device (size, format, image quality) and is no longer the same as the original artwork. The only way to avoid this "duplication" is to not use local artwork, which the OP seems happy with (until the remote artwork is no longer available, of course).

However, a "global" actors folder might still be possible, but would need to be implemented by Ember.

For instance, Ember could populate a single folder (located somewhere in the media library, ie. Movies/.actors) with actor artwork for *all* movies (or it could be an actor folder per source).

There'd be nothing for Ember to download if an actor is already present in this "global" folder. Use a, b, c etc. sub-folders a la Thumbnails to avoid 10,000 image files in a single folder. You could even use a hash as the filename. Whatever, these are implementation details...

Ember would then store this "global" local actor artwork location in the NFO - this is the tricky bit, as the actor artwork url would need to be in nfs://, smb://, Z:\, /var/media, etc. format as it would be Kodi eventually reading and caching the artwork, and Kodi would need an absolute path to the "global" folder.

Then, when Kodi imports the NFO, it would import all the local actor artwork from this "global" actor folder - all the benefits of local artwork, but without the duplicate actor artwork (same actor, different movies etc.) splatted throughout the media library.

Obvious downside is that it would be very easy to "break" actor artwork by altering the "global" actor folder location. "Locking" the absolute path into the NFO would make switching from nfs:// to smb:// difficult (the NFO would need to be rewritten with the updated path), and using a mixture of nfs:// and smb:// to refer to the same media library would be impossible.

One final downside is that anyone crazy enough to export their library from within Kodi would still create individual .actor folders alongside each movie, but I can't see why anyone would want to do this when using Ember.

Last thing, actor artwork would need to be in the correct orientation (see my previous post on this).

HHHmmmm!!!
ok not being a techie.. I just need to know if my assumptions are correct.
Background: I have 5 instances of Kodi running 14.2 release.
my library is on a NAS
all 5 instances sharing the same library and MYSQL also on NAS
have substituted the path of Thumbnails, playlists, sources.xml and mediasources.xml on NAS being shared by all instances.
if I scrape the movie media with all options excluding the actors with Ember and then use local NFO for populating the library in Kodi will Kodi get the actor thumbs while adding to library and place them in the thumbs folder?? should this not avoid duplication??
all other movie related artwork is in the movie folder.
also except for my main Kodi installation where I scrape and update library the other instances are all residing on small (256GB) SSD Windows 7 mini PC's
any simple input on this will be appreciated...thanks...