Upgrade to 2.14 fails due to a failed database migration

Expected behaviour

When upgrading from 2.13 to 2.14 all migrations should take care of known issues that will cause schema migrations to fail and result in a successful migration.

Current behaviour

When upgrading from 2.13 to 2.14 the changes introduced by THREESCALE-9655 cause a migration to fail.

Migrating to AddPrimaryKeyToFeaturesPlans (REDACTED)
== REDACTED AddPrimaryKeyToFeaturesPlans: migrating =====================
[strong_migrations] DANGER: No lock timeout set
-- execute("ALTER TABLE features_plans ADD CONSTRAINT features_plans_pk PRIMARY KEY (plan_id, feature_id)")
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::UniqueViolation: ERROR:  could not create unique index "features_plans_pk"
DETAIL:  Key (plan_id, feature_id)=(32, 13) is duplicated.

The cause of this is clear in the logs and is in fact due to a bug on the API that allowed duplicate entries of the same record to exist. That bug has in fact been fixed indirectly by the introduction of a primary key on this table but the migration does not take care of the fact the bug existed and would have gotten databases into a broken state in the first place.