  We study the possibility of obtaining unbound energy E_{c.m.} in the centre
of mass frame when two particles collide near the inner black hole horizon. We
consider two different cases - when both particles move (i) in the same
direction or (ii) in the opposite ones. We also discuss two different versions
of the effect - whether an infinite energy can be released in the collision
(strong version) or the energy E_{c.m.} is finite but can be made as large as
one likes (weak version). We demonstrate that the strong version of the effect
is impossible in both cases (i) and (ii). In case (i) this is due to the fact
that in the situation when E_{c.m.} formally diverges on the horizon, one of
particles passes through the bifurcation point where two horizons meet while
the second particle does not, so collision does not occur. In case (ii), both
particles hit different branches of the horizon. The weak version is possible
in both cases, provided at least one of particles starts its motion inside the
horizon along the direction of spatial symmetry from infinity.
