You could try and change the connectionString section to look like this:

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

Remove the old key, then add the new one.

I too encountered this same error. I had a .info environment created with DNN, which was all ready to move to the root directory (to replace a .org environment).  I am still having issues with this task, but all along the .info site remained intact, until about 2 weeks ago when I first encountered this "'SiteSqlServer' has already been added." error message.  The hosting company's support was of no help, to be honest, and I spent several hours trying to get my known-good .info site back on-line.  Then I found this discussion thread...

Yes, through my mucking around, I ended up with two web.config files, a default DNN one in the root directory, and the (correct/working) web.config file in the .info directory.

So, as an experiment, and with nothing to lose, I added the <remove name="SiteSqlServer" /> in the (correct/working) web.cofig, exactly as Viraj indicates above, and to my joy, low-and-behold, I got my .info site back online.  What a relief!

Thank you very much Viraj!

Now it's back to trying to get everything DNN operating in the root...(sigh).

Jack