I noticed you had Arid Mesa in there but no basic mountains or plains which would make it useless unless cards like Volcanic Island counts as technically they are both types. I also noticed that you have many lands that sacrifice themselves for a specific color or two specific colors I think it would be beneficial to add some three color lands like Frontier Bivouac, Opulent Palace, Crumbling Necropolis or Savage Lands. Rupture Spire and Grand Coliseum are good too. Unknown Shores is another option if you still need more multicolored.

Myrothios I agree with your suggestion regarding excessive use of fetch lands which may be detrimental for keeping life totals in check in game scenarios. Lands like Reflecting Pool may be a better alternative. However, as for the other lands you suggested, the reason I opted for untapped lands is because of the aggressive nature of Duel commander. You cant actually afford to have lands come to play tapped except necessary utility lands like Bojuka Bog

What about Eternal Witness, it may help you fetch some useful resource that you self-milled.