Too common, unfortunately. Publishing and getting public credit like this are considered high stakes (which justify, or at least make some sense of occurring despicableness). There is much too much infighting over nothing (compared to the money in the corporate world).

Hard to say but my impression is that most academics are honest and would try not to do this, but also there are rivalries between labs and that tends to encourage "everything they do is bad and we're great" mentalities so it's definitely not surprising.

As common as pedophilia in priests: IOW, despite anyone's "feel" or "gut impression", probably the exact same distribution as in the general population.

Nothing about belief in the RC church nor education of the priests filters for pedophilia, despite lots of loose opinions. Priests, plumbers, and people who live in Scarsdale are all generally equally likely to be pedophiles. (There are meaningful filters, like man versus woman.)

Nothing about pursuing an academic career selects for or filters against dishonesty. I've seen astounding dishonesty in published papers; I've seen admirable examples of morality as well.