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 `<remove>` tags like this for any offending elements:



<pre><code>&lt;system.webServer&gt;
  &lt;modules runAllManagedModulesForAllRequests="true"&gt;
<b>
    &lt;remove name="ErrorLog" /&gt;
    &lt;remove name="ErrorMail" /&gt;
    &lt;remove name="ErrorFilter" /&gt;
</b>
    &lt;add name="ErrorLog"    type="Elmah.ErrorLogModule, Elmah"    preCondition="managedHandler" /&gt;
    &lt;add name="ErrorMail"   type="Elmah.ErrorMailModule, Elmah"   preCondition="managedHandler" /&gt;
    &lt;add name="ErrorFilter" type="Elmah.ErrorFilterModule, Elmah" preCondition="managedHandler" /&gt;

  &lt;/modules&gt;
&lt;/system.webServer&gt;
</code></pre>

In Visual studio, Solution->Web.Project->Properties->Web, I have changed my Project Url from     `http://localhost:51123/` to `http://localhost:51123/NewProjectName` and I keep getting this error:

"*Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to ....*" on every module added. 

Adding a remove tag works but then it should have been a problem even before i changed the url. Any suggestions?