I personally record my activities on my phone and also on my cheap GPS device as a backup, which I manually upload through the website in case the Strava app crashes.
Many people I know also record on their phone and another device, such as a Garmin or Apple Watch.
Problem is, most of these devices automatically upload the activity to Strava once it's finished, and when the person also finishes the activity in the Strava app, it creates a Group activity which double counts the activity.
Sometimes this is unintentional, but sometimes it's blatant cheating.
