Google Spreadsheet + GAS (Google Apps Script) onEdit Executing Twice

Overview

When using GAS (Google Apps Script) to add processing when editing a Google Spreadsheet, there were cases where the onEdit function was executed twice upon editing the spreadsheet.

Cause and Solution

The cause was that the onEdit function was also selected as a trigger for spreadsheet editing.

Since onEdit is a reserved function, there was no need to set up a trigger for it.

Summary

I hope this is helpful for anyone experiencing the same issue.