diff --git a/hogwarts/courselist_test.go b/hogwarts/courselist_test.go
index 2cecbf4..aab0205 100644
--- a/hogwarts/courselist_test.go
+++ b/hogwarts/courselist_test.go
@@ -14,6 +14,9 @@ func impl(t *testing.T, prereqs map[string][]string, courseList []string) {
 				t.Errorf("course %v (index %v) depends on course %v which is not taken yet", course, index, prereq)
 			}
 		}
+		if learned[course] {
+			t.Errorf("course %v (index %v) has been taken at least twice", course, index)
+		}
 		learned[course] = true
 	}
 	for course := range prereqs {