Fixed value discount added twice is calculate discounts button used

1. Create a Fixed discount with a code. In our case its limited to one role.
2. Enter the code in the discount box in the check out pane
3. Click 'calculate discount'
4. Discount is calculated and applied. Appears in totals box
5. Click 'review order'
6. Discount is applied again. Appears twice in order summary box. This can result in order total going negative.

If you don't click 'calculate discounts' discount is applied once when you click 'review order'.

The discount should only ever be applied once. If its calculated when you click 'calculate discounts' it should not be applied again.