If a PC is being “restrained” by a creature - could the restraining creature also in turn be “restrained” by a different PC / creature? And, would doing so break the initial restraint on the PC, or would all of them now be stuck in a restrain-chain of sorts?
Yes, but it would continue to restrain its target
The restrained condition has these effects (PHB, p. 292):
- A restrained creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.
- Attack rolls against the creature have advantage, and the creature’s attack rolls have disadvantage.
- The creature has disadvantage on Dexterity saving throws.
None of them prevents the creature from becoming restrained itself. None of them blocks the creature from restraining another creature. So you could restrain the creature, but it would not break the initial restraint on the PC.
Another supporting proof is the Grappler feat: its pin feature restrains both you and the grappled creature, which would not work if you could not restrain a creature while restrained. (Thanks to @Kryomaani).