Supervisors constantly rejecting my ideas. What to do?

When I was in medical devices we had lab books to support potential patent claims etc..

Write your ideas in a lab book.

If it happens again, you could try to say something like, 'I think I may have a communication issue. I suggested this idea on xxx but it appears I may not have communicated the concept very well. Perhaps I'm letting myself down in communicating my ideas very well. If it happens again could you let me know so that I can better express myself.'

You've taken it on the chin, asked them for help in a non academic way and brought to attention your original idea. Everytime they do it, you can remind them in this manner, they'll have to take you seriously eventually; if they continue to dismiss your ideas and then suggest them later, you'll have a strong record for challenging that behaviour etc..

Very importantly be respectful, they may just find it hard to understand you.

There are several possible explanations for this. First, maybe your advisors couldn't fully understand your ideas because you are still not fluent in the jargons of the field. Second, (as suggested by Steven Gubkin in the comment), it is possible that your idea and your advisor's idea are actually different, but the difference is perhaps too subtle for you to tell. Third, there could be a status issue going on, where new students' ideas tend to not be taken seriously. Fourth, your advisors could simply be forgetful, and did not realize that the ideas they are proposing were once suggested by you.

All of these explanations are, unfortunately, quite common. Regardless, it is important for you as a student to learn to articulate clearly your ideas and to defend your ideas. It may also help to talk with other students who have worked with your advisors to see if what you are experiencing is a more persistent/serious problem; if so, it may be a good time to find some less abusive advisors.

Yes, you might have a communication problem. Hopefully that will improve over time as you are still new at this.

But, if the "new" idea appeals to you, I suggest that you swallow your pride and run with it. Never mind that someone else thinks they thought of it first. It is possible that your earlier suggestion is what put that thought into their head.

I had a somewhat similar situation when writing a book and reviewers kept asking for changes. Eventually we went around a circle and wound up where I started. I was happy, they were happy. (Well, I complained to the editor, but the pride was strong in me, then.) Anyway that is where it was left and the book got published.

If the "new" idea doesn't appeal to you then you are in the same situation. I'd suggest that, instead of presenting your ideas orally, you write them up as best you can and use that writeup as the basis of any meeting. Perhaps it will make things clearer to the supervisors. And, having those notes can also be the basis of future work when you are more independent.

But remember, for now, that the main task is to get to graduation by an acceptable path in reasonable time. Don't fight when you don't need to.