'Abdullah bin 'Amr used to say:"The Messenger of Allah forbade offering more for something that has already been bought by his brother, or for a man to propose marriage to a woman when someone else has already proposed to her, unless the previous suitor gave up the idea or gave him permission."
أَخْبَرَنِي إِبْرَاهِيمُ بْنُ الْحَسَنِ، قَالَ حَدَّثَنَا الْحَجَّاجُ بْنُ مُحَمَّدٍ، قَالَ قَالَ ابْنُ جُرَيْجٍ سَمِعْتُ نَافِعًا، يُحَدِّثُ أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ نَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يَبِيعَ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلاَ يَخْطُبُ الرَّجُلُ عَلَى خِطْبَةِ الرَّجُلِ حَتَّى يَتْرُكَ الْخَاطِبُ قَبْلَهُ أَوْ يَأْذَنَ لَهُ الْخَاطِبُ .
Nesai
The Book of Marriage • Hadith 3243