What is the ruling on performing a supererogatory prayer during the 'Asr time, and is it permissible to interrupt a supererogatory prayer to catch the congregational prayer with the Imam?

Repeating congregational prayer is a matter of scholarly disagreement. The preponderant view among the Shafi'is is that it is always recommended to repeat it. However, if the congregational prayer is called while one is performing a supererogatory prayer, the preponderant view is to make it brief and complete the supererogatory prayer quickly.