DT work helpWatch
From my personal experience, choose a design movement that is completely out there and different from the standard. My project was achieved A* (pre-number GCSEs) and my exam was A* too, and I used the Danish Modern Movement as design inspiration rather than the typical art Deco or Nouveau. If you are genuinely concerned with someone using your design, bring it up with your teacher as it is an issue with integrity.
Feel free to fire me any questions!
Plagiarism is a serious matter and even though they seem to be copying from you there is no way to prove to an examiner you havent copied from them. Although its nice to help people out ultimately this is your qualification and you need to put yourself first.
Be more careful about sharing your designs, often products made in a school environment will be quite similar if you are working towards a similar brief with similar materials and equipment available to you, they shouldnt be so similar that dimensions are exact between the two.
You're never going to create something totally original in a GCSE environment (hell even in a professional environment truly unique products are few and far between) but you dont need to. You just have to add a value to something that already exists and this value will be your unique selling point compared to other things.