There is a bug with the ghost component code gen that has appeared after updating from Netcode for Entities 1.9.0 → 1.9.1. If any component/buffer/rpc includes two Entity fields that have the [GhostField] attribute it will produce this error in the temp netcode generated file for it:
A local variable or function named 'ghostComponent' is already defined in this scope
It can be easily reproduced in any project with Netcode for Entities 1.9.1 by adding a simple component like this:
public struct MyEntitiesComponent : IComponentData
{
[GhostField]
public Entity Entity;
[GhostField]
public Entity AnotherEntity;
}