Django Validate M2M Form Save

Question

I am trying to creating cron application my model look like :

class Crontab(models.Model):
    #Some filelds 
    ...
    ...
    #m2m fields From here prob starts
    minute = models.ManyToManyField(Mst_Cron_Minute, db_table="crontab_minute_map")
    hour = models.ManyToManyField(Mst_Cron_Hour, db_table = "crontab_hour_map")
    day_of_month = models.ManyToManyField(Mst_Cron_Day_Of_Month, db_table="crontab_day_of_month_map")
    month = models.ManyToManyField(Mst_Cron_Month, db_table="crontab_month_map")
    day_of_week = models.ManyToManyField(Mst_Cron_Day_Of_Week, db_table="crontab_day_of_week_map") 
    description = models.CharField(max_length=250, blank=True)
    is_active = models.IntegerField(choices=YES_NO_CHOICES)
    reason_for_deactivate = models.CharField(max_length=250, blank=True)

I need to validate the cron before it saves it into database.

1. Should not have duplicate cron data saved on this model.
2. if already a data in db say 5 1 * * * then for new record in the position of * none of the value allowed. Example 5 1 * * 1 or 5 1 * 1 * or 5 1 1 1 1 not allowed.
3. if i have a data data 5 1 * 1 * is in DB the new data * 1 * 1 * or 5 * * 1 * ... not allowed, it mean if there is a digit in the position the * not allowed.

How can i validate this ? where my code should go ? Please help me .