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functions and variables defined in userscripts are not made available in the global page scope

This is not true.

The way userscripts work is via script injection (yes, it's basically an attack). The only way userscripts can get access to variables and functions in the page is to expose userscripts to the page - thus the page has access to the userscript.

Therefore the main reason to use an IIFE in userscripts is to avoid messing up scripts running on the page.

Some script injection system may transparently execute your userscript in an IIFE behind your back (this is what nodejs does with modules - yes, it's not a userscript system but it is an example of software that does this). In such cases you don't need to manually code the IIFE yourself. I personally don't know which extension does this and which does not so I tend to include the IIFE just to be safe.

If your code does not define any new variables or functions you also can get away with not using an IIFE because there is nothing in your code that can override existing code.

Simply replace your 2 $push in group stage by $addToSet : it will 'push' values only once, without dupicates