Did you see this - http://kb.discountasp.net/KB/a799/error-accessing-wcf-service-this-collection-already.aspx
You can resolve this error by changing the web.config file.
With ASP.NET 4.0, add the following lines to your web.config:
<system.serviceModel>
<serviceHostingEnvironment multipleSiteBindingsEnabled="true" />
</system.serviceModel>
With ASP.NET 2.0/3.0/3.5, add the following lines to your web.config:
<system.serviceModel>
<serviceHostingEnvironment>
<baseAddressPrefixFilters>
<add prefix="http://www.YourHostedDomainName.com"/>
</baseAddressPrefixFilters>
</serviceHostingEnvironment>
</system.serviceModel>