Jerry,
I believe that 3 is the minimum number of duplicate pairings. To achieve this, the duplicates must all occur in the foursomes. So this does make for a somewhat unfair schedule as one player is always in the foursome and three players are always in the threesomes:
(1 2 3 5) (4 8 9) (6 7 10)
(1 2 4 6) (3 8 10) (5 7 9)
(1 3 4 7) (2 9 10) (5 6 8)
Ian.