Clear blocked hosts when a role duplicate task is found in free strategy

In the free strategy, we mark a host as blocked when it has work to do
(the PlayIterator returns a task) to prevent multiple tasks from being sent
to the host. However, we check for role duplicates after setting the blocked
flag, but were not clearing that when the task was skipped leading to an
infinite loop. This patch corrects that by clearing the blocked flag when
the task is skipped.

Fixes #15681

lib/ansible/plugins/strategy/free.py

@@ -123,6 +123,7 @@ class StrategyModule(StrategyBase):
                             # if there is metadata, check to see if the allow_duplicates flag was set to true
                             if task._role._metadata is None or task._role._metadata and not task._role._metadata.allow_duplicates:
                                 display.debug("'%s' skipped because role has already run" % task)
+                                del self._blocked_hosts[host_name]
                                 continue
 
                         if task.action == 'meta':