Detect duplicate property values in placeholders/mixins

Add support to the `DuplicateProperty` linter to detect duplicate
property values in placeholder and mixin declarations.

Also cleaned up some Rubocop violations.

Change-Id: I6ed7b5daace7ebf453b4f56460a1eab58f3a84de
Reviewed-on: http://gerrit.causes.com/36672
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>