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I have seen colleagues present the same data analysis multiple times over several years.

Each time, what the study is supposedly "about" is different, sometimes even using different theories and claiming to have a new research question. But the data analysis remains pretty much the same, maybe some new control variables or a different regression technique.

This is explained as having the previous paper rejected at a journal and needing to change the "framing" of the paper.

Is the behavior of my colleagues ethical?

What venue are they “presenting” the work in, and does it carry an expectation of originality?

Publishing multiple minor variations of the same piece of work would indeed generally be considered as unethical, or at best borderline-ethical “salami-slicing”. That’s because publishing carries an expectation of originality/novelty: you’re not supposed to publish the same work twice. This is deeply entwined with the ways that publications are used as a metric of productivity. However, it sounds like this isn’t what’s happening here, if your colleagues haven’t succeeded in publishing the work.

Whether presenting multiple minor variations of the work is ethical depends entirely on what venue they’re presenting in (major conferences? informal workshops? the departmental seminar?) and whether it has an expectation of originality like publications would.

This varies between fields and subfields, and between conferences within subfields. In pure maths (at least in the subfields I’m familiar with), conference presentations have no expectation of originality. You can present the same piece of work every year for a decade, and people won’t be very excited or impressed, but it won’t be considered as unethical. In computer science, on the other hand (again, in the subfields I know), presentations at major conferences are like a smaller version of a journal publication, and as such, are explicitly required to be original: re-using material between them would be unethical. But in less formal settings (e.g. local workshops), again there’s no such expectation of originality, and re-use of material (with or without variation) is fine.

It is difficult to say whether it is ethical or not; it also will differ based on what the conferences say (e.g., do you have to explicitly state that the work has not been presented elsewhere?). There is also a gray area here: There is nothing wrong, for instance, with a secondary analysis of previously collected data.

But a colleague who has nothing new to tell for years, has probably a much bigger problem anyway...