Hi, and welcome to the Apple (user-to-user) Community.
Re: "iPhone alarm's "Stop" button acts as the "Snooze" button
–how can I ensure the alarm doesn't go off a second time?"
We do not know which iPhone model you are using, or the iOS version that is installed:
Generally ... it is how you set the alarm that reveals whether the alarm stops or "snoozes".
ie: If not set to "Snooze" it should not repeat.
Eg: For iOS 16 (or please choose an earlier iOS at the top of the article for accurate info):
Set an alarm on iPhone - Apple Support
says:
"Set an alarm
You can set alarms for any time, including one for the time you want to wake up.
Any alarm you set up in Clock is unrelated to a sleep schedule.
- Tap Alarm, then tap .
- Set the time, then choose any of the following options:
- Repeat: Choose the days of the week you want the alarm to repeat.
- Label: Give the alarm a name, like “Water the plants.”
- Sound: Choose a vibration, song, or ringtone.
- Snooze: Give yourself a few more minutes of sleep.
- Tap Save.
To change the alarm, tap the alarm time. Or tap Edit at the top left, then tap the alarm time."
"Turn off an alarm : Tap the button next to the alarm time".
All the best :-)