Duplicate key error on switching services

[ph448]

This seems to only happen to services switching multiple times, we get:

IntegrityError at /switch_services/352/
duplicate key value violates unique constraint "apimws_ansibleconfiguration_service_id_57c01285a0290788_uniq"
DETAIL:  Key (service_id, key)=(703, backup_first_date) already exists.

This means that https://github.com/uisautomation/mws-panel/blob/0abdf8602b25527e8896f9e3ff9974d84d968e59/mws/sitesmanagement/models.py#L286-L287 fails.