A believer is the brother of a believer, so it is not lawful for a believer to outbid his brother, and he should not propose an engagement when his brother has thus proposed until he gives it up.  

أَنَّهُ سَمِعَ عُقْبَةَ بْنَ عَامِرٍ عَلَى الْمِنْبَرِ يَقُولُ إِنَّ رَسُولَ اللَّهِ ﷺ قَالَ الْمُؤْمِنُ أَخُو الْمُؤْمِنِ فَلاَ يَحِلُّ لِلْمُؤْمِنِ أَنْ يَبْتَاعَ عَلَى بَيْعِ أَخِيهِ وَلاَ يَخْطُبَ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَذَرَ  

[Machine] The Messenger of Allah ﷺ said, "A believer is a brother of another believer. It is not permissible for a believer to sell over the sale of his brother until he leaves it, and he should not propose to the woman who is engaged until he leaves her."  

قَالَ رَسُولُ اللهِ ﷺ «الْمُؤْمِنُ أَخُو الْمُؤْمِنِ لَا يَحِلُّ لِمُؤْمِنٍ أَنْ يَبْتَاعَ عَلَى بَيْعِ أَخِيهِ حَتَّى يَذَرَ وَلَا يَخْطُبَ عَلَى خِطْبَتِهِ حَتَّى يَذَرَ»  

A believer is the brother of a believer, so it is not lawful for a believer to outbid his brother, and he should not propose an engagement when his brother has thus proposed until he gives it up. (Using translation from Muslim 1414)   

أَنَّهُ سَمِعَ عُقْبَةَ بْنَ عَامِرٍ عَلَى الْمِنْبَرِ يَقُولُ إِنَّ رَسُولَ اللهِ ﷺ قَالَ الْمُؤْمِنُ أَخُو الْمُؤْمِنِ فَلَا يَحِلُّ لِمُؤْمِنٍ أَنْ يَبْتَاعَ عَلَى بَيْعِ أَخِيهِ حَتَّى يَذَرَ وَلَا يَخْطُبُ عَلَى خِطْبَةِ أَخِيهِ حَتَّى يَذَرَ