When a new form submissions is received, the Google Forms add-on will send one email message per rule. In some cases, you may observe that duplicate email messages are sent for the same rule.
This likely happens when more than one onSubmit trigger have been added to your Google Form or when multiple form collaborators are managing the form rules for the same Google Form.
Restart Google Forms Trigger
Open your Google Form that is sending double notification emails and launch the forms add-on. Choose
Forms Troubleshooting from the menu and click the
Restart button from the troubleshooting window. This will internally cleanup extra triggers associated with your form.
Once the addon has been restarted, submit a new form response and then open your Gmail Sent Items folder to confirm that only one email notifications has gone out for the submitted response.
Disable Notifications for Form Collaborators
You may also receive double email notifications if your other team members have also enabled the addon for the same Google Form from their own Google accounts. Ideally, only the form owner should be responsible for managing the rules for their Google Form.
When multiple people enable form rules, the notification emails will go out from their email accounts as well resulting in duplicate notifications. You can compare the emails sent by the form and look at the From field - if it contains the address of another team member, they need to disable the form rules activated at their end.
To rectify the issue, contact your team member and ask them to disable notifications at their end. Please note that this should only be done under the email account that is sending the duplicate notifications.