getEntityModel on @ExernalEntity anntotated class fails when called more than once

Description
Suppose we have the following:

public class ThirdPartyEntity {
    public String field;
    public Long number;
}

and

@ExternalEntity(target = ThirdPartyEntity.class)
public class ThirdPartyEntityMixIn {
    public String field;
    @Id
    public Long number;
}

When calling datastore.getMapper().getEntityModel(ThirdPartyEntityMixIn.class), the entity model of ThirdPartyEntity is returned correctly the first time. However, on the second call, getEntityModel fails due to a duplicated discriminator error:
Two entities have been mapped using the same discriminator value (com.example.ThirdPartyEntityMixIn): com.example.ThirdPartyEntity and com.example.ThirdPartyEntity

Cause
The issue occurs because when entities are being put into mappedEntities, the key used is entityModel.getType(), which refers to ThirdPartyEntity.class. But when retrieving the model in getEntityModel, the key used is ThirdPartyEntityMixIn.class, leading to a conflict when Morphia attempts to register the entity again.

Morphia Version:
2.4.4

Start by reproducing the issue with the ThirdPartyEntity and ThirdPartyEntityMixIn examples, calling getEntityModel twice. Inspect getEntityModel and the mappedEntities registration and lookup paths. Done means repeated calls no longer trigger the duplicate discriminator error and return the expected entity model.