روى ابن عمر رضى الله عنهما ان رسول الله صلى الله عليه وسلم نهى ان يبيع  بعضكم على بيع بعض ، ولا يخطب الرجل على خطبة اخيه حتى يترك الخاطب قبله او ياذن له الخاطب

On the authority of Ibn Omar(May Allah be pleased with them)that the messenger of Allah(May  thePeace and Blessings of Allah be upon him)banned   a person from buying something that has already been  sold to another person and banned a man from engaging a girl that has already been engaged to another man unless the first suitor gives her up, or allows him to ask for her hand .