IIS express: Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to

Question

In Visual studio, Solution->Web.Project->Properties->Web, I have changed my Project Url from http://localhost:51123/ to http://localhost:51123/NewProj

Answer 1

All web.config files work off multiple cascading levels of inheritance at the machine, IIS, project, and folder level locations, with each providing a higher degree of specificity.

If you're getting this error, it means you've either:

1. Added the same key twice in the same file (unlikely since you would've seen it)
2. The same key already exists in a separate file higher up the inheritance chain

There can be a lot of different root causes for #2, but if you want to side step them, you can just remove any previous declarations and then re-add your own at that level (I'd pay good money for an upsert feature).

So just add tags like this for any offending elements: