Some kind of rim reflections, maybe from the filter stack? Also possible you got the reflections on the inside of the filter(s), ie. between rear filter surface and the front of the lens. Flare induced by the finder typically has a more defined outline and often shows line patterns.
Double filters usually would not be considered a good idea anyway, at least for visible light work. (filter stacks are commonly used for UV photography, but the reasons for this are irrelevant here).