Problem: using MailApp.sendEmail in a script to send an email as the result of a Google Form subscription. Worked fine in scripting environment, but when tested from the form, would result in two copies sent to the address specified in the script, and copies appearing in both the script account sent mail and inbox.
Fix: Switching to GmailApp.sendEmail resulted in correct behavior (one email to address, copy only in sent mail).
Full thread https://github.com/dwyl/learn-to-send-email-via-google-script-html-no-server/issues/129