Extend DuplicateRoot to MergeableSelector

Extends the behavior to check nested rule sets in addition to root-level
ones.

Change-Id: I251dd13ef6d9d2a3ef504be3e699ead5ce81b929
Reviewed-on: http://gerrit.causes.com/37604
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>