The following players have played for both Celtic and West Ham ;
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I can think of nine - Jimmy Delaney, Pat Crerand, Lou Macari, Brian McClair, Lee Martin, Liam Miller, Dion Dublin, Roy Keane and Henrik Larsson.
There may be others.
I'm guessing there'll be a few in the history books, but two off the top my head are Rio Ferdinand and Paul Ince.