Cannot add gift item to cart

Description

Precondition:
A gift promotion: "Buy over $100 and get gift variant A".

Steps to reproduce:

1. Add items to a cart with price greater than $100
2. Variant A is added to cart with price $0.
3. Continue to add variant A to the cart.

Expect: Variant A can be added to the cart normally with its original price.
Actual: Cannot add variant A to the cart.

Note:

• The issue happens in
  • Quicksilver sample site
  • Commerce Manager of Quicksilver
  • Commerce Manager of Seagull
• The issue does NOT happen in
  • Seagull sample site