malloc double free behaviour

Question

The GNU manual page for malloc defines that when free() called twice with the same pointer (that was previously allocated by malloc())

Answer 1

"Undefined behavior" simply means that the implementation isn't required to deal with the situation in any particular way; any behavior or result is considered "correct".

The result of free-ing a pointer more than once is up to the underlying implementation of malloc; the result could be a crash and core dump, or it could corrupt the malloc arena (fouling up future allocations/deallocations), or it could ignore the double free entirely.