diff --git a/golem_activity/models/golem_activity.py b/golem_activity/models/golem_activity.py
index 7f0a504..b9a26e8 100644
--- a/golem_activity/models/golem_activity.py
+++ b/golem_activity/models/golem_activity.py
@@ -102,6 +102,14 @@ class GolemActivity(models.Model):
                     activity.date_start > activity.date_stop:
                 raise models.ValidationError(_('Start of the period cannot be '
                                                'after end of the period.'))
+            if activity.season_id.date_start > activity.date_start < activity.season_id.date_end:
+                msg = _(u'Start of the period is in range of an '
+                        'existing period.')
+                raise models.ValidationError(msg)
+            if activity.season_id.date_end < activity.date_stop > activity.season_id.date_start:
+                msg = _(u'End of the period is in range of an '
+                        'existing period.')
+                raise models.ValidationError(msg)
 
     @api.onchange('season_id')
     def _onchange_season_dates(self):