I created the mtg-duels-pc-game tag a while back.
I added on the "pc-game" bit because of a related issue I've observed on another site. On RPG.SE, we have a [dnd-encounters] tag. D&D is a popular game, and it prominently features fights we call "encounters", and sometimes a person asking about encounters in D&D will use the [dnd-encounters] tag. However, the [dnd-encounters] tag is actually for a specific official organised play program called "D&D Encounters", so we retag these questions to use another tag like [encounter-design].
I added "pc-game" to head off this kind of situation from the outset. MTG is, after all, regularly played using duels.
That said, calling it "mtg duels" when it's called "magic duels" is definitely wrong. (At the time I thought it was called "Magic: the Gathering — Duels" or something.)
I'd agree with magic-duels now though. That sounds OK. We might have to do some retagging but probably not very much.
If we find ourselves doing a lot of retagging, we may want to rename it to magic-duels-video-game.