Don't allow people to proxy a path to itself, or to another proxy.

middleman-core/lib/middleman-core/sitemap/extensions/proxies.rb

@@ -31,6 +31,8 @@ module Middleman
           # @param [String] target
           # @return [void]
           def proxy_to(target)
+            target = ::Middleman::Util.normalize_path(target)
+            raise "You can't proxy #{path} to itself!" if target == path
             @proxied_to = target
           end
       
@@ -55,7 +57,11 @@ module Middleman
               proxy_resource = store.find_resource_by_path(proxied_to)
           
               unless proxy_resource
-                raise "Path #{path} proxies to unknown file #{proxied_to}"
+                raise "Path #{path} proxies to unknown file #{proxied_to}:#{store.resources.map(&:path)}"
+              end
+          
+              if proxy_resource.proxy?
+                raise "You can't proxy #{path} to #{proxied_to} which is itself a proxy."
               end
 
               proxy_resource.source_file