Help me I.D. this sci-fi book ...

It’s actually a series of four books, but they all happen at the same time. I read them about fifteen years ago, so they’re at least that old, but I don’t think a whole lot older.
The premise is that there are four planets around one sun, and each planet has some sort of virus or something that mutates anyone who visits the planet. This mutation, which is different on each planet, kills the person subjected to it if they try and leave. The planets are therefore used as a giant penal colony, since the criminals have no hope of escape.
The plot revolves around some sort pf conspiracy happening on the planets. Since a spy can’t go to the planets and return, the authorities clone one of their agents four times, and send one clone to each planet, and then somehow extract his experiences back into the original agent.

Any ideas on what this might be?

“Four Lords of the Diamond” by Jack L. Chalker. One of my favorites.

They don’t really “clone” him. They take 4 real prisoners each headed to a different planet. They wipe their memories and replace them with those of the protagonist.

The “mutation” is a kind of intelligent viral infection which has similar but distinct properties on each planet, amounting to a sort of magic with a mechanical backbone. A Chalker trademark.

Cool, thanks Latecomer. It’s been awhile since I read them. I’ve looked in bookstores, but never could find them again.

FDF - scifi book club also had this come out in an omnibus by that title- you might want to keep an eye out for that too.