I've defined the following two models:

class Weekday(models.Model):
    _name = 'ludwik.offers.weekday'
    name = fields.Char()

class Duration(models.Model):
    _name = 'ludwik.offers.duration'
    weekday = fields.Many2many('ludwik.offers.weekday')

When I try to start odoo I get the following message:

ProgrammingError: relation "ludwik_offers_duration_ludwik_offers_weekday_rel_ludwik_offers_" already exists

Also, when I change the _name properties in models, the problem persists (of course the relation name in the error message changes to reflect the rename), so it's not like it's just a conflict with some old relations already existing in the database.