Is there any particular reason the procedure for washing hands for bread includes pouring the water twice on each hand? Isn't once enough?
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The basic idea is that the first splash of water becomes tamei from touching your hands (since they themselves are deemed tamei - that's the reason we have to wash in the first place); the second splash, then, makes the water left on your hands tahor. If your hands were dirty, you actually need three splashes: one to remove the dirt, and the second and third serve as the necessary two splashes.
Actually, the Shulchan Aruch in OC 162:2 (agreed upon by the Rama) rules that you need to splash once per hand plus:
I find that when using less than 86.4 mL per hand it is exceedingly difficult to cover the entire hand with the water. Additionally, a standard washing cup holds about a liter (about 12 revi'its) so the vast majority of people do not need a second splash the way we are accustomed to washing.
I note that the Mishna Berura there (sk 21) notes that it is possible that the Gra holds like the Rashba who requires two splashes always, but the Mishna Berurah agrees that the majority opinion is as I quoted above.