It is the easiest thing to do.
You get an interesting new result [a new technique or a new system] and then you publish a paper.
Now, there is lots of "low-lying fruit".
There are a few loose ends to tie up and so your write another paper providing a bit more evidence the first one was correct.
Or you apply a slightly different technique or probe to your new system.
Or you apply your new technique to a slightly different system.
Sometimes this may be reasonable, or even important.
But, other times this recycling may just reflect our laziness, lack of originality, or succumbing to the pressure to add more lines to our CV.
This issue was first brought to my attention when I was a postdoc. A research fellow suggested to me that each person in the group basically had a single paper they were "republishing". This shocked me. I am not sure this was fair but I have not forgotten the concern.
Later, a colleague was evaluating Professor X and told me he thought that "every paper X wrote was the the same." On reflection, I think this was quite harsh. X had a developed a powerful technique that they had applied to a range of systems. The technique was not easy to use and often produced definitive results. In contrast, other scientists X was being compared to might publish on a more diverse range of subjects, but not produce definitive results. Like Galileo, I think the former is more valuable.
We need to consider whether we are vulnerable to such criticism, even if it may be unfair. Unfortunately, perceptions do matter.
But, we should also ask whether it would be better if we moved on to something else, or at least diversified. Perhaps we should leave others to lie up the loose ends or take the next steps. I suspect that is what great scientists do.
I welcome suggestions of critieria to help decide when "enough is enough".