[Victor Stinner email@example.com]
The PEP 8001 is not trivial, it expects a specific format:
**DO NOT LEAVE ANY BRACKETS BLANK!** **DO NOT REPEAT A RANKING/NUMBER!**
[Nathaniel Smith firstname.lastname@example.org]
I'm not sure what the motivation for those restrictions is. I guess with IRV there isn't an obvious way to handle a repeated number,
FWIW, I've never seen any IRV variant that allowed duplicate rankings. And I don't see how it could: at each round it's counting only the number of _first_ ranked candidates among those who remain, and if a voter had ties for their first place candidates that voter would be helping multiple candidates survive the round. "Not fair."
but with Condorcet it's no problem. And both methods are fine with leaving some options blank (it's equivalent to ranking the blank options as tied for dead last).
But leaving some candidates unrated is common in IRV schemes. When the last of a given ballot's rated candidates is eliminated, that ballot is thrown out and the election continues as if it had never existed (in particular, it no longer counts for the "majority" needed to win the next round).
PEP 8001 says:
""" A vote which omits candidates in the ranking is invalid. This is because such votes are incompatible with the desired properties listed above, namely:
Making voters consider alternatives, as well as Doing everything possible to reach a conclusion in a single election. """
The first point is dubious. While it may be hard to prove, what will happen in reality is that some voters will simply make up rankings for PEPs they never even read. This is so common in forced-ranking systems that it has a name:
There may be some merit to the second point, and especially if donkey-voting is common: the more people mindlessly give a rank equal to a candidate's distance from the top, the less likely cycles are to form (assuming all ballots list the PEPs in the same order - which they really shouldn't, but probably will ;-) ).
In any case, without IRV there's scant reason to require that all ranks be distinct, and I think dropping that requirement would be a serious improvement, both for making it easier to construct a valid ballot, and especially to allow voters more possibility to express their true preferences (including "these two, three, ... are equally fine by me").
What may be hard to get across then is that, e.g., the ballot rankings
1 1 1 6 6 6
and 5 5 5 6 6 6
have exactly the same effect: "I like the first three better than the last three - maybe a little, maybe a lot, you just can't tell from what I'm allowed to express."