Yow. I didn’t even realize the first three were possibilities.
@AlexandraIDV Opening the Tampermonkey dashboard will check for 1. I think in Firefox Tools → Add-ons and themes will let you check for 2-4.
Re: 5, the one console.log call already added appears to show the script is only being run once, which is what has me so mystified.
Thanks for the reply!
Regarding this, have you checked if the callback in the then handler of the fetch request is only executed once, or did you just check if the main script is only run once? I can’t think of any scenario where it would be called twice on one fetch with the default options, but it might be a good idea to check just to make sure.
I tested the script with the same setup (Firefox on Windows 10, same scripts) and I only get the output once. I even tried to see if I could get the then handler to run twice, but I could only get the script to execute once.
My instructions above were to add the console.log() call inside of the then handler, not inside the main script. I’m pretty sure those instructions were followed exactly (based on her console screenshot).
I’m trying and failing to understand how the <section> could be added twice without that console.log() also being called twice, though. If there were a page refresh in between the then() calls, there would still only be one <section class="tofugu-latest">, right?
I think Kumirei’s thoughts around multiple independent instances of the script are the only thing that make sense (one version of the script with the console.log() and the other without).
So weird.
If a page refresh occurs the entire page will be reset, and tampermonkey will re-run all scripts from the start. Any scripts executing on the previous instance of the page will be terminated before the new one starts loading, so a refresh shouldn’t affect your script.
That would be one option, I wonder how that would happen though. Tampermonkey should prevent double installations. By default, if a script is already installed and you try to install it again, it will reinstall instead of add, so you’ll still be left with a single instance. I don’t know what the behavior would be with 2 script managers though, although I assume that would give problems during installation too. It’s quite hard to get a double install by accident 
But possibly less difficult than inserting the section twice while only logging to the console once.
@AlexandraIDV: These are the only further debugging steps I can think of:
Open the Tampermonkey dashboard and check that:
Check the currently loaded extensions in Firefox. This should be under the Tools menu, Add-ons and Themes. I think it’s accessible via CTRL-Shift-A on a PC. Ensure that only Tampermonkey is currently enabled.
Maybe check for the latest version of Firefox and upgrade if required.
Hopefully, one of these steps will uncover the problem.
You could also add a check for the element before adding and, if it exists, either not do anything or replace it
I’ll add that as a bit of “belt and suspenders” programming in the next version.
I’d still want to know WHY it’s already there, though. Something isn’t right if it’s already present.
Yikes. I just looked at this thread for the first time in a long, long while, and it appears I dropped the ball.
Did the double-loading problem ever resolve itself? I can add some debugging to try and resolve the issue if not.