BUG: ss2tf error with double integrator

[roryyorke]

Describe your issue.

Initially from python-control/python-control#715 (comment)

ss2tf adds an extra, very fast stable zero for double integrator systems.

Reproducing Code Example

import numpy as np
from scipy import signal
p = np.array([ -2, 0, 0])
z = np.array([ -1])
k = 1
bb, a = signal.ss2tf(*signal.zpk2ss(z,p,k))
print(np.roots(bb[0]), np.roots(a))

Error message

# not an error message, but the output; expect [-1], [-2, 0, 0], but get:
[-7.50599938e+14 -1.00000000e+00] [-2.  0.  0.]

SciPy/NumPy/Python version information

1.9.1 1.23.1 sys.version_info(major=3, minor=10, micro=0, releaselevel='final', serial=0)