MobyGames [1] claim they are unique missions. More or less the same story, but different maps, objectives and briefing text.

Maybe I need to re-download it, and check out the differences. I remember playing those six missions so many times before eventually saving up enough pocket money to buy the game, but I don't exactly remember them being different.

And it's actually six maps, three for each faction.

[1] https://www.mobygames.com/game/57961/warcraft-ii-tides-of-da...