"None of you should propose marriage to a woman when someone else has already proposed to her, unless he marries (and he gives up the idea), or gives him permission." (Using translation from Nasāʾī 3241)   

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «لَا يَخْطُبُ أَحَدُكُمْ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَنْكِحَ أَوْ يَتْرُكَ» وَقَفَهُ مُحَمَّدُ بْنُ سِيرِينَ  

[Machine] A man should not compete with his brother's bid, nor propose to his brother's betrothed. Hisham raised it.  

«لَا يَسُمِ الرَّجُلُ عَلَى سَوْمِ أَخِيهِ وَلَا يَخْطُبُ عَلَى خِطْبَةِ أَخِيهِ» رَفَعَهُ هِشَامٌ  

It was narrated from Abu Hurairah that the Prophet said: "None of you should propose marriage to a woman when someone else has already proposed to her." (Using translation from Nasāʾī 3240)  

عَنِ النَّبِيِّ ﷺ قَالَ «لَا يَخْطُبْ أَحَدُكُمْ عَلَى خِطْبَةِ أَخِيهِ»