Fine, they may accidentally be covering the same material with the same assignment, but why should the students have to do the same work twice?
I was just thinking, I did the opposite of this when I was a teacher. I taught physics and I taught Space Science. These two classes covered many of the same things, for example when talking about Newton's laws and Kepler et al. I know at least a few times I gave the exact same assignment. It was applicable in both contexts and had I had a student in both classes I would have only made her only do the assignment once.
In engineering, if a student didn't reuse her work from another class in my class (and I found out about it). I would very likely mark her down for being stupid.