Hi,
In your Web.config you may find something like this:
<staticContent>
<remove fileExtension=".mp3" />
<remove fileExtension=".mp4" />
<remove fileExtension=".m4v" />
<remove fileExtension=".m4a" />
<remove fileExtension=".ogg" />
<remove fileExtension=".ogv" />
<remove fileExtension=".webm" />
<remove fileExtension=".oga" />
<remove fileExtension=".spx" />
<remove fileExtension=".eot" />
<remove fileExtension=".otf" />
<remove fileExtension=".woff" />
<remove fileExtension=".svg" />
<remove fileExtension=".svgz" />
<mimeMap fileExtension=".mp3" mimeType="audio/mp3" />
<mimeMap fileExtension=".mp4" mimeType="video/mp4" />
<mimeMap fileExtension=".m4v" mimeType="video/mp4" />
<mimeMap fileExtension=".m4a" mimeType="audio/m4a" />
<mimeMap fileExtension=".ogg" mimeType="video/ogg" />
<mimeMap fileExtension=".ogv" mimeType="video/ogg" />
<mimeMap fileExtension=".webm" mimeType="video/webm" />
<mimeMap fileExtension=".oga" mimeType="audio/ogg" />
<mimeMap fileExtension=".spx" mimeType="audio/ogg" />
<mimeMap fileExtension=".svg" mimeType="image/svg+xml" />
<mimeMap fileExtension=".svgz" mimeType="image/svg+xml" />
<mimeMap fileExtension=".eot" mimeType="application/vnd.ms-fontobject" />
<mimeMap fileExtension=".otf" mimeType="font/otf" />
<mimeMap fileExtension=".woff" mimeType="font/x-woff" />
</staticContent>
However if those settings are set at a higher level in IIS and locked down then settings like this in Web.config can cause errors. In this case the error message is complaining about .mp4 is a duplicate of settings higher up in IIS so you should remove this one:
<remove fileExtension=".mp4" />
<mimeMap fileExtension=".mp4" mimeType="video/mp4" />
if you get a different error after that then keep removing the ones it complains about, but don't remove the ones it doesn't complain about.
Hope that helps,
Joe