1GB used issue in twitter winning

I receive a twitter dm today on my username @qabuketholwa2 from @mtnza as they cornfirm as i win 1Gb data with pin voucher code if i try to load on my number ********** 645 it show already used then i call mtn customer care today to confirm about this issue the agent told me to read voucher and said it was used on 06 May how could this happen please help me to solve this matter as im glad but mtn play with my feeling today