Obviously there are a few issues here.
I DO understand where you are coming from.
As an example, when I got engaged, many moons ago, my mother gave me a 5 piece saucepan set but kept the largest one for herself! I see the funny side now but at the time I was very hurt.
I felt undervalued and hurt that neither I nor the rather important occasion merited a whole present and was seen as an opportunity to get something for herself. No, it was nothing to with the actual saucepan.
Firstly, I would strive to see the funny side, which it seems like you're already doing. It really does have a funny side.
Secondly I would accept that you won't change them. A totally honest open, serious request that they stop would really involve you saying the reasons behind it. How do you dress up or say kindly that you feel they are thoughtless and cheap? You can't. They would be very hurt and it could really harm the relationship you do have.
So resign yourself to the fact that these lovely gifts will keep coming. Then, I would designate a particular place in your house, a drawer or something, perhaps even give it a name, and as soon as you can, deposit the unwanted item in to it.
Use it or wear it a few times in their sight if you need to, if you can, then put it back. Then, a bit like when someone dies, after a 'respectable' period, say 6 months, or if you are really guilted about it, a year, donate it.
In fact you could keep last year's birthday/Christmas present till you have the next one to replace it with giving you a full year until donation time! Generally even if you were using it, it would acceptable to stop after a year.
But don't just ignore your hurt feelings about it. This is only something you can work through yourself. Your worth and your value are not reflected in or dictated by, by cheap gifts. Do you really feel that is true? If not, why not?
They do not automatically imply that your parents don't value you as they should. They say more about your parents than you and not necessarily in a negative way. They could be viewing this all from a totally different angle e.g. truly believing it does not matter if it is the EXACT thing if it's sort of what you asked for, the know you'll like it because it's similar.
They may want to not just get what you've told but be seen to be bothered enough to find something similar. They may genuinely think 'what is the point of paying £X for that when this is basically the same thing much cheaper?'
None of the above may be right but the point is they are thinking in their own groove not yours.
If it really is a sign a sign that they don't care 2 hoots about you then you have to address that and work on how you feel about yourself.