The property or navigation cannot be added to the entity type because a property or navigation with the same name already exists on entity type
public class Device
{
[Required]
[DataMember(Name = "key")]
[Key]
public Guid Key { get; set; }
[ForeignKey("DeviceType")]
public Guid DeviceTypeId { get; set; } // I assumed primary key of your `DeviceType` entity is `Guid`
[ForeignKey("ModelType")]
public Guid ModelTypeId { get; set; } // I assumed primary key of your `ModelType` entity is `Guid`
[IgnoreDataMember]
public virtual DeviceType DeviceType { get; set; }
[IgnoreDataMember]
public virtual ModelType ModelType { get; set; }
}