The alerts which has been already deleted still go off?
In that case, please do the following.
Method1: Restart your device
Just restart your device.
*After restart the device, please once unlock your phone. So the app can start the repairing process.
Now deleted-alerts won’t ring any longer.
Method2: Force stop & repair alert
If you don’t want to restart your device, then do this method.
Long press Reminder FLEX icon and select info.
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Press Force stop
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Reopen Reminder FLEX > Settings > Repair robot (Alert Repair)
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Press REPAIR ALERTS
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Now, your problem should be resolved.