Sorry I forgot that was there. judging by this line:
AbsoluteURLReferrer:http://foo.bar.org/Admin/Extensions/t...
it looks like that file is already installed on the website.......
if you want to try this go for it, but I don't know how it will affect your other sites.... also did you try to install this skin on this site before?
If you go into your ftp hosting site go to foo.bar.org>Admin>Extensions>tabid>1951>ctl>Install>rtab>1951 and select the file named Default.aspx copy it and then delete it... and try reinstalling.... IF it is not located there you might have to search around for the file....

Well I seem to have at least found the root cause of the issue.

When /Admin/Extensions loads it calls the following stored procedure:

exec sp_executesql N';Exec dbo.GetPackages @0',N'@0 nvarchar(4000)',@0=NULL

The procedure parameter is @portalID and is being passed as null, so it returns modules and skins on every site. I think from site admin mode, it should pass the current portal ID?

I think this procedure is then used to check to see if this install is a new one or an upgrade of an existing item. In this scenario (null portalID) it finds the skin in the list 2x from the other sites (it shouldn't see it at all) and results in the "Sequence contains more than one matching element" error. If the skin is only installed on one other site, the error doesn't get thrown, but it is still an issue, because the system thinks the module is installed on that site and is probably trying to run the install as an upgrade.

I created an account on dnntracker.atlassian.net hoping I could report this officially, but I don't have rights to add an issue. How can I get this reported?