You're not alone if your iPhone alarms look like this:
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It's understandable - you don't want to oversleep, so you set multiple alarms. Or you had to wake up really early one time to make a flight, so you created a new alarm for that, too.
After a while, your iPhone alarm clock can get messy with scores of alarms you're no longer using.
The easiest, and to my knowledge, only way to clear them all at once is to use Siri.
Hold down your iPhone's home button, and tell Siri: "Delete all my alarms." It looks like this:
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After you press "Confirm," Siri will say, "OK, no more alarms."
Be careful - you'll need to set new alarms if you don't want to oversleep.