It appears that Snowdon/Eryri/Yr Wyddfa is STILL simultaneously in England and in Wales, being as it is the highest peak in both of those fair countries according to nearly every description of it I read and hear (most recently on BBC R5 this am). Schrodinger's Mountain. It's a marvellous thing when you think about it. Is it unique? I don't know of any other mountains that manage this feat.