One of my colleagues told me that one of his papers was rejected for presentation at a conference. One year later a paper was accepted with almost the same content at the same conference.
Is it in some ways preventable? Does a preprint ensure against such idea copying?
There are two aspects. If the author of the new paper saw the old one and copied it, then it is clear plagiarism and a conference might have ways to deal with that, provided it can be proven and is brought to their attention.
But, ideas are free. In some fields, a lot of people are thinking the same thoughts simultaneously as the state of the art is open to all. It is possible that the new paper was done independently of the first one, in which case there is no issue. No one "owns" ideas. Ideas are free. For such a case, no, it isn't preventable and is normal.
A middle ground might occur if the new paper acknowledges (cites) the old one, in which case this is just normal academic progress.
If the paper has appeared on a preprint service then you have a claim on priority but not "ownership of the ideas". You still don't know whether the other author actually saw and copied the paper or developed it independently. But it would reduce the novelty of their paper, given your priority.
