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http://www.math.sfu.ca/~alspach/mag86/
1 to 166 for a four-player game, not changing much when more players join the table.
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That's not what the article says. It says in a 10-handed game, where only 2 players have been dealt pocket pairs preflop, its 1 in 1035 to be set over set on the flop.
It says in 10-handed games where 2-4 players have been dealt distinct pocket pairs preflop, it is 1 in 166 to have set over set on the flop.
Of course, the condition to have multiple pocket pairs dealt preflop is not always present.
Bottom line, if you flop a set, there is far less than a 1% chance someone else has too.