Is there anything from the series that strongly suggests tulpas and doppelgangers are different beings? Instinctively, I assume they are different, but maybe I am overlooking something.
Well, according to the series (not necessarily definition) it seems that Tulpas are manufactured replicas and as far as I can tell, doppels are organic "mirror selves."
However, if you look at definitions, especially of Tulpas, I think they are interchangeable, one and the same. But since Tulpas disappear and turn to gold ball when they enter the lodge and it seems that Doppels do not, it cannot be said they are the same.
However, one Tulpa definition says "A Tulpa is a thoughtform, or being created from the collective thoughts of separate individuals. The concept of Tulpas is theoretical in nature and originates from Tibetan mythology, where Tulpas are described as extra bodies that were created from one person's mind in order to travel to spiritual realms."
They are not created by a lock of hair but by the collective thoughts of different people. However, there are other definitions too.
So the conclusion is: Dunno.
According to the first two seasons, doppergängers (apparently) don't need the golden seed.
Fire walk with