The bug
You can attach a lead to a dolphin when a lead is already attached to it. Not a duplicate of MC-96347.
Steps to reproduce
Attach lead to dolphin
attach lead to fence
Attach lead to dolphin
With any other mob it doesn't let you.
If you are in survival mode, it destroys the original lead.
Attachments
Comments 4

Can confirm in 1.17.
Can confirm in 1.17.1.

Can confirm in 24w14a.
Can confirm this happens, and with Axolotls too
MC-227553