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

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

@@ -36,14 +36,14 @@ module Middleman
       @opts = opts
 
       @locs.each do |k, _|
-        if self.respond_to?(k)
-          msg = "Template local `#{k}` tried to overwrite an existing context value. Please renamed the key when passing to `locals`"
+        next unless self.respond_to?(k)
 
-          if app.build?
-            throw msg
-          else
-            logger.error(msg)
-          end
+        msg = "Template local `#{k}` tried to overwrite an existing context value. Please renamed the key when passing to `locals`"
+
+        if app.build?
+          throw msg
+        else
+          logger.error(msg)
         end
       end
     end