TicketExistsError when issuing tickets

[ricwo]

Rasa version: 1.5.X

Issue:
A TicketExistsError can lead to the lock store getting stuck in a loop trying to issue a new ticket

Error (including full traceback):

rasa-production_1  | rasa.core.lock_store.TicketExistsError: Ticket '11874' already exists for conversation ID '8e001a2806b1489897accc755dd9b24f'.
rasa-production_1  | 2019-12-06 15:13:45 ERROR    rasa.core.lock_store  - Ticket could not be issued. Issuing new ticket and retrying...
rasa-production_1  | Traceback (most recent call last):
rasa-production_1  |   File "/build/lib/python3.6/site-packages/rasa/core/lock_store.py", line 109, in issue_ticket
rasa-production_1  |     self.ensure_ticket_available(lock)
rasa-production_1  |   File "/build/lib/python3.6/site-packages/rasa/core/lock_store.py", line 251, in ensure_ticket_available
rasa-production_1  |     "".format(existing_lock.last_issued, lock.conversation_id)
rasa-production_1  | rasa.core.lock_store.TicketExistsError: Ticket '11874' already exists for conversation ID '8e001a2806b1489897accc755dd9b24f'.
rasa-production_1  | 2019-12-06 15:13:45 ERROR    rasa.core.lock_store  - Ticket could not be issued. Issuing new ticket and retrying...
rasa-production_1  | Traceback (most recent call last):
rasa-production_1  |   File "/build/lib/python3.6/site-packages/rasa/core/lock_store.py", line 109, in issue_ticket
rasa-production_1  |     self.ensure_ticket_available(lock)
rasa-production_1  |   File "/build/lib/python3.6/site-packages/rasa/core/lock_store.py", line 251, in ensure_ticket_available
rasa-production_1  |     "".format(existing_lock.last_issued, lock.conversation_id)
rasa-production_1  | rasa.core.lock_store.TicketExistsError: Ticket '11874' already exists for conversation ID '8e001a2806b1489897accc755dd9b24f'.