To make sure you don't double add friend pairs you would need a rule for deciding which of any two friends goes in the first key field and which goes in the second key field.
...assuming that friendship is commutative. (i.e., "If Alice likes Bob, then Bob likes Alice.") If it's directional, as on LiveJournal or Twitter, then this would not apply, since "Alice likes Bob" and "Bob likes Alice" are independent of each other.

In reply to Re^2: best way to keep track of who's friends with whom by dsheroh
in thread best way to keep track of who's friends with whom by keiusui

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