Today, I happened on a fun quirk of web-dev. What happens if you include the same script twice, say like this:
<!DOCTYPE html> <html lang="en"> <head> <script src="test.js" type="module"></script> <script src="test.js" type="module"></script>
test.js only runs once, despite being included twice. It seems modules have a specific trait where they're only ever evaluated once, I believe specified in steps 4 and 5 of ECMAScript 6.0 section 188.8.131.52.5.
Nothing good ever comes of running scripts multiple times. I'm glad it's out.