Add DuplicateProperty linter

Add linter `DuplicateProperty` that can detect when a property is
declared multiple times in a single rule set.

Change-Id: I048a8240aee28e0bfda8e7ab0ec8b65f595f698f
Reviewed-on: https://gerrit.causes.com/29755
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Tom Dooner <tom.dooner@causes.com>