Ships with multiple "Give Medical Assistance" missions will cancel the others when one is accepted.

[Ben Lubar]

However, if you rescue a ship's crew from a derelict before any mission is generated and the NPC shuttle picks them up from your ship and you manage to accept the "Give Medical Assistance" missions before the next shows up, you can have multiple concurrently active "Give Medical Assistance" missions and they work fine.

[Darke]

I've noticed it and just assumed the code was built off the "retrieve object from derelict" mission code, where also it blocks off the other ships when you pick one of them. :/