Duplicate validation message in some discount types

Description

Steps to reproduce:

1. Create a discount of the type "Spend for Discounted Selection", "Discount off Total Order Value", or "Discount off Shipping Costs".
2. Input all required fields.
3. Select the "Amount off" option but don't input data.
4. Save the discount.

Expected: You see a message "At least one discount amount must be entered."
Actual: You see a duplicate message "At least one discount amount must be entered."