Hi,
Normally exam discount vouchers are only valid for a single use. So if you would like to re-take the exam you would pay full price, unless you obtain another voucher.
For questions related to exams/vouchers/discounts/etc., please ask your question in the Microsoft Credentials forum using link below. There are support moderators over there that will respond to your question in 1 business day and can assist:
https://trainingsupport.microsoft.com/en-us/newthread?threadtype=Questions&forum=mcp
Please click Accept Answer and upvote if above was helpful.
Thanks.
-TP