This bug only occurs on spigot, vanilla commands don't cause this. I initially discovered this when players where kicked for spam while messaging each other (using Essentials) despite messages being infrequent (10 seconds apart) and containing different text. Server operators are not affected by this bug.

 Steps to recreate this bug from a clean install of Spigot 1.13 with no plugins:

1. Make sure the player is not op
2. Type out /version
3. Press and hold down the space bar for a few seconds
4. Quickly press enter to execute the command.

This bug also occurs while typing out any sort of message command with small words, such as typing long messages in /msg with Essentials. It only happened with longer messages that were about a sentence in length or at least 12 words, and did not occur when using the up key to repeat a previous message. After each word I type, it gives a suggestion of the current online players. Perhaps this queries the server and is what is causing the spam. Could there be a way to disable this command suggestion for plugin commands while the user is typing out the command, or go back to the 1.12 method of tab completion? Even if I turn off command suggestions in the minecraft client, this bug still occurs.