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Bill McLean (32)
I think there has been a few; probably not at the level of the league world cup.
Before the rule changes I can think of one; Ilivasi Tabua.
Ilie as he was called represented Australia at the Sevens RWC in 1993 and then the RWC in 1995. He then represented Fiji at the RWC in 1999.
I do also know that Semi Taupeaafe (Waratahs) represented Australia at the Sevens RWC in 1993 and then Tonga at the 1995 and 1999 RWC's. The same happened to an NZ/Samoa representative called Junior Paramore.
Then the rules changed and have been solid ever since. I know there was a push to alter them in 2009 but it was vetoed by IRB who did not want to see a farce like the RLWC with players moving between Tier 1 and Tier 2 nations.
I remember this because there were several ex-All Blacks at the time (most of whom were playing in Europe) looking to play for pacific island teams at the 2011 RWC.