FWIW, I had the se occurrence in my sheep draft, where it was a "pick two players from one match" round. If blocked, they had to pick players from the same match. In some cases, a match only had three eligible players, so if managers blocked each other, they screwed themselves a bit because they would block each other again on a second pick before opening up the pool again.
That would seem fair, mainly because the managers went for a GOAT player, one of the best in the draft, and there's a risk associated with that.
Otherwise some kind of "middle ground" could be found, in which they don't get the whole pool, but only players from the same WC that their other pick was from (if there are any?) but that's just an example.