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In page Robert de Vere, 9th Earl of Oxford:

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Robert de Vere was the only son of Thomas de Vere, 8th Earl of Oxford and Maud de Ufford.[1] He succeeded his father as earl in 1371, and was created Marquess of Dublin in 1385. The next year he was created Duke of Ireland. He was thus the first marquess, and only the second non-princely duke (after Henry of Grosmont, 1st Duke of Lancaster in 1337), in England. King Richard's close friendship with de Vere was disagreeable to the political establishment.[citation needed] This displeasure was exacerbated by the earl's elevation to the new title of Duke of Ireland in 1386.[2] His relationship with King Richard was very close and rumoured by Thomas Walsingham to be homosexual.[3]