How do I get a separate grade for a Skill I assign twice?

In the past, I feel like I have been able to assign the same skill twice and Khan Academy recognizes it as two separate assignments.  Now, when I assign a skill once with the same questions and then again (right after) with different questions, my students are receiving a grade for both as soon as we do the "same question" assignment together.  I even have different due dates on them but that isn't helping.  What am I missing?