In some better institutions there is cooperation amongst the teachers and teachers know what other teachers teach and therefore they guide and direct their own instruction accordingly. They don't repeat things that other teachers do, and they try to make some sort of a coherent whole among all of the material that is taught.

In other institutions where there is less cooperation among the teachers or where the cooperation comes from the top down then there is more overlapping.

In the case of the cooperation from the top down the department chairman may tell the teachers what they have to teach and assume that based on his overall knowledge of the entire system there will be no overlapping. Of course, this is not the case and teachers, once they get into the classroom, are going to be overlapping with others unless they talk among themselves. If they talk among themselves there will also be overlapping, but that's even less important than the fact that the students who take the courses will or will not hear the same material twice from two or more points of view, but more importantly they will not understand why they are studying anything that they are studying.