Only discounts with highest priority are applied in certain circumstances

Description

This is a fix for an issue where only discounts with highest priority are applied in certain circumstances.

Steps to reproduce (sample site):

1. Set PromotionExclusionLevel to "Unit".
2. Create two discounts of type " Spend for Discounted Selection " in two different campaigns. The discounts should target different products. 
3. Set both discounts to exclude each other. 

Expected result is that a customer can get both discounts. 
Actual result is that only the discount with highest priority is applied.