Prevent duplicate role insertion into roles: (#50552)

* Corner case in which import_role would add another instance of a role with the same signature into roles: when it already existed there.

  roles:
	- name: a
  tasks:
       - import_role: name=a

  would execute role 'a' 3 times instead of the intended 2 (x2 in roles: phase +1 in tasks:)

* added tests

changelogs/fragments/fix_ir_dupes.yml

@@ -0,0 +1,2 @@
+bugfixes:
+    - prevent import_role from inserting dupe into `roles:` execution when duplicate signature role already exists in the section.