It's possible to add the same content twice to Schedule list.

Steps:
1. Create new Landing Page.
2. Add Schedule block.
3. Change template to list.
4. Add content item.
5. Try adding the same item again.
Actual: It's possible, item is visible twice in preview (attached).
Expected: It's not possible, error message is displayed that block already contains item.

Remarks: This was addressed before here https://jira.ez.no/browse/EZEE-540.