This rarely happens. For most users, the following solutions work:
1. Restart the iPhone. If restart did not work for you, consider force restarting.
2. Remove Case (if it is thick and rugged). Try back tap once by removing the case.
3. Check/Change Back Tap Settings: Open the Settings app → Accessibility → Touch → Back Tap. Now, tap Double Tap and choose a different action. (Do not choose 'Shake'). Now, check if the double back tap performs this new actions or not. Do the same for Triple Tap. This should hopefully fix things.
4. Check for iOS update: If you have a pending update, please open the Settings app → General → Software Update → Download and Install.
5. Reset All Settings: At last, if nothing works, please open the Settings app → General → Reset → Reset All Settings. Note: This will not delete personal data like apps, music, photos, videos, etc. But all settings you have changed or added will be restored to default.
Hope the above solutions help.