Problem with Forms Authentication with AD

I am trying to set up my Sharepoint website to use forms authentication with AD. I followed the instructions inside here: http://msdn.microsoft.com/en-us/library/ms998360.aspx

However when I try to login using my credentials, I get an error message:

Error: You were not connected because a duplicate name exists on the network. Go to System in Control Panel to change the computer name and try again.
 (C:\Inetpub\wwwroot\wss\VirtualDirectories\TestSite80\web.config line 120)

 My membership configuration inside Web.config looks like this:

 <membership defaultProvider="MyADMembershipProvider">
      <providers>
      <add
   name="MyADMembershipProvider"
   type="System ...