From my understanding, while the Plague Furnace does have the keyword Plague Priest, the model itself is not a Plague Priest as what the conditional text on the Liber Bubonicus calls for. I cannot benefit from doubling up on Pestilent Prayers, but it can replace Noxious Prayers with Pestilent Prayers.
If there is a ruling to allow for the Plague Furnace to get two rounds of Pestilent Prayers, that'd be excellent. Unfortunately I can't find anything on it.
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