Letting you reuse Treasure or tap trinkets like Food and Clues for mana is absurd, especially given how free those tokens seem to be these days
Haven of the Spirit Dragon, Crucible of the Spirit Dragon, and Maelstrom of the Spirit Dragon are lands that can produce mana support for dragons, while offering some solid land-sac abilities
Fortunately, Skysovereign offers some indirect ways to mitigate multiples
Ive now been in more than a few games where the best thing you can do to move a game along is to make a clone of an opposing Orcish Bowmasters, just to take it off the battlefield
Bane of Progress comes with a bonus elemental to dole out damage, but Wave of Vitriole can actually be more detrimental to your opponents' health in the long run
This extremely potent mana reduction can allow you to start double or even triple-spelling much earlier in the game, and push your opponents to keep up with you or lose