Relation R is an antisymmetric-relation if for distinct x and y, R(x,y) implies not R(y,x). In other words, for all x,y, R(x,y) and R(y,x) => x=y. R(x,x) is still possible.
(<=> (Antisymmetric-Relation ?R) (And (Binary-Relation ?R) (=> (And (Holds ?R ?X ?Y) (Holds ?R ?Y ?X)) (= ?X ?Y))))