For your first error, this happens because every time a wounded player would normally die, the death gets cancelled by the plugin, but the death hooks are called anyways. Since I cannot prevent hooks from getting called, there is not much I can do on my side. The best solution would be for the developer of MedicalItems to implement check wether an entry in the dictionary already exists before inserting a new one.
The reason for your second issue is pretty much the same, I cannot do much on my side I, but starting with the next version I will provide an API where plugins can check if a player is actually just died or not.
I don't quite understand what you mean by "allowing pvp damage in this situation". If damage to a wounded player would be allowed, they could die before a medic arrives. This would make the entire medic system unnecessary.