I think it's very unlikely for mw.hook to fire the same events multiple times. But perhaps the code in Special:MyPage/minerva.js is executed more than once, so your handles are registered more than once?

Yes, you're absolutely right. Special:MyPage/minerva.js is executed twice. Updating title and description accordingly.

If it is of any help, the first one comes from load.php?debug=false&lang=sv&modules=mobile.usermodule&only=scripts&skin=minerva&user=Nirmos&version=0zycl1h:1

14 ms later another one comes from VM2919:1

MobileFrontend has its own 'mobile.usermodule', loaded in SkinMinerva, which loads this script. That's the first one you're seeing.

I don't see anything in there to prevent the core 'user' module from being loaded (which loads the User:<skin>.js for the current skin, which is Minerva on mobile). I suspect that results in it being loaded again.

The reason for this is probably change 06ab9c0942f66317b20130ed12e677607783a61e in MediaWiki core, which allowed the 'user' module to load on mobile sites.

'mobile.usermodule' (MobileUserModule class) is documented as follows:

/**
 * Alternative of ResourceLoaderUserModule for mobile web.
 * Differs from the user module by not loading common.js,
 * which predate Minerva and may be incompatible.
 */

However, I can see my own common.js being loaded on the mobile site. If my guess about the reason is correct, this has been happening for half a year now, and I haven't heard anyone complaining about this. I think the solution here is simply to remove 'mobile.usermodule' (and 'mobile.usermodule.styles').

Will investigate soon. See also T147755: Uncaught Error: Unknown dependency: site.