This is relating to SPIGOT-5172 however it wouldn't let me add on the video/plugin I wanted to share, as well as update that this still occurs in 1.14.4.

Information:

• Does not appear to work on minecraft commands, only plugin ones (ALL plugins).
• Using EssentialsX, typing "/give diamond" will execute the command once typing forwards, but if you hit backspace up until your command line says "/give" will make the event run twice.

 I've attempted to locate an issue within the code but have not had success, probably because I'm bad.

Video showing the full issue (too big to upload here): https://youtu.be/IxO80U72roQ

Plugin: Provided in Attachments below