So this was quite a hilarious error and was quite difficult to trace, so I'm going to chuck everything on here in case it happens to anyone else;
While working on a website in production, its name changed. I noticed that the IISExpress instance displayed not only the new name, but a '\' and the old name as three separate instances.
As I was attempting to add log4net to the project, I attempted to use this line in the web.config:
<log4net configSource="log4net.config" />
Which caused the following error on app run:
the configSource file '[file]' is also used in a parent, this is not allowed.
the configSource file is also used in a parent, this is not allowed.
This was perplexing as I had not used this source anywhere else, and no matter what file I changed it to, I got the same error.
log4Net worked fine when I defined it entirely within my web.config, but that's not what I wanted, so I investigated further. Interestingly, a colleague who'd just been brought onto the project could run the app fine with the separated config, but I and another dev who'd been working on this project for a while could not.
How can I and another colleague with exactly the same setup have a failing app with the same config as someone with a fresh checkout have it work fine?
It turned out to be that IISExpress had saved the previous settings in a .vs folder at solution's root level, and all three of those instances were using the same config, and looking for the same file, hence the error.
The solution was as follows:
\ instead of \newname\).vs directoryFor different versions of VS, the .vs folder is in different locations on your machine.