There have been situations when the server or Internet connection was slow and form users didn’t wait for the submission to be sent. To prevent duplicate form submissions you can set your Submit button to disappear after it was clicked. Usually, this options is set by default. Even though the connection is slow, the form users can not click the Submit button again.
Go to the Edit Form section, select the Submit button (Send Email, Send), below, and tick Disable Submit button after click, in the Edit Field panel.