The entry 'ConnectionStringName' has already been added.

http://dev.communityserver.com/forums/t/478332.aspx

Try this:

    <connectionStrings>
        <remove name="ConnectionStringName" />
        <add name="ConnectionStringName" connectionString="server=(local);uid=;pwd=;Trusted_Connection=yes;database=communityserver" />
    </connectionStrings>