Sayyiduna Uqbah bin Amir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "When two guardians give (a woman) in marriage, the marriage by the first of them is valid; and when a man sells goods to two men, it belongs to the first buyer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13794