Students should be aware that teachers can be lazy (like everyone else), and may repeat exam questions. It's an important part of exam strategy to teach that -

Students should be aware that teachers can be lazy (like everyone else), and may repeat exam questions. It's an important part of exam strategy to teach that - by telling students. But this is a hack, as you say: it's not learning the material. If you don't have the understanding, then this hack is not valuable and may be very dangerous to rely on. You are not preparing the students for an appropriate diversity of exam questions if you ask the same questions each time. The students are perhaps doing worse because they have understood, probably correctly, that your testing is not a worthwhile exercise for them. If you don't have novel questions, you are only demonstrating that it is unimportant to learn new material, or unimportant to find a deeper understanding of the existing material.