polresultant disagrees with sage, maxima and magma

[Georgi Guninski <guninski@guninski.com>]

I don't claim this is a bug in pari, more like a bug in the
mentioned CAS.

? p1=x2*(x3-x4);p2=x2*(x3-2*x4);polresultant(p1,p2,x1)
%1 = 0

Since p1 and p2 certainly have common roots I expect the resultant
w.r.t. x1 (not present in p1 or p2) to be able to vanish.

sage, maxima and magma return $1$ on the above testcase.

Can't see how it can be both ways, which is correct?