Let A extend B and A#foo() override B#foo(). When validating an invocation of A#foo() the current implementation will evaluate all parameter constraints defined at A#foo() and B#foo(). That way foo()'s preconditions defined in B are strengthened by A.
According to the "Programming by contract" article on WP this is not allowed, subtypes may only weaken preconditions defined by supertypes. The common implementation pattern for this is to combine the preconditions within a hierarchy by a logical OR, meaning the weakest precondition in the hierarchy applies.
Note that postconditions (return value constraints) may be strengthened (but not weakened) by subtypes. Therefore the current implementation (AND combination) should be correct here.