Sayyiduna Uqbah bin Amir al-Juhani (may Allah be pleased with him) says that the honoured Messenger (peace and blessings be upon him) said: "Any woman whom two guardians marry off, the marriage by the first is the valid one."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13803