The Eastern Churches of Orthodoxy have a history of heresy. There are really too many to mention all of them here but you can start with Arianism, and finish with the polytheism of Photian monopatricism from Blachernae. Following Palamas, they have a divine uncreated essence and a divine uncreated energies and they can both be worship as god but they aren’t the same. In other words they created a second god to explain their Filioque rejection. Really we probably should make all of them be rebaptized since it’s a different god they worship.