Throw when trying to overwrite a template context value. Fixes #1884

middleman-core/lib/middleman-core/template_context.rb

@@ -36,7 +36,8 @@ module Middleman
       @opts = opts
 
       @locs.each do |k, _|
-        if self.respond_to?(k)
+        next unless self.respond_to?(k)
+
         msg = "Template local `#{k}` tried to overwrite an existing context value. Please renamed the key when passing to `locals`"
 
         if app.build?
@@ -46,7 +47,6 @@ module Middleman
         end
       end
     end
-    end
 
     # Return the current buffer to the caller and clear the value internally.
     # Used when moving between templates when rendering layouts or partials.