This is a hard one to troubleshoot without knowing more, but I suspect it’s because you have another alarm set – either part of another routine or a separate alarm.
I would check to make sure that there aren’t any alarms set first – and check any other Amazon accounts you have in your household also for the same thing.
To check your own account for alarms, open the Alexa app, select “More” at the bottom and then “Alarms and Timers”. Delete any extra ones that may be overlapping. Do the same thing with Routines, to make sure there aren’t any overlapping ones.
Next, check the phones of any of your other family members in the household. There may be some overlapping alarms or routines between the two accounts.
That would be the best way to uncover where this problem is coming from – hope you find the problem alarm soon!