Warn when a duplicate key is found in a .strings file.

Author: nolanw

R.swift/ResourceTypes/LocalizableStrings.swift

@@ -69,6 +69,10 @@ private func parseStrings(stringsFile: String, source: String) throws -> [String
         }
       }
 
+      if dictionary[entry.key] != nil {
+        warn("Skipping duplicate value in \(source) for key: \(entry.key)")
+      }
+
       dictionary[entry.key] = (params, entry.val)
   }