  Recent simulations and observations suggest that star clusters form via the
assembling of smaller sub-clusters. Because of their short relaxation time,
sub-clusters experience core collapse much earlier than virialized
solo-clusters, which have similar properties of the merger remnant of the
assembling clusters. As a consequence it seems that the assembling clusters
result in efficient multiple collisions of stars in the cluster core. We
performed a series of $N$-body simulations of ensemble and solitary clusters
including stellar collisions and found that the efficiency of multiple
collisions between stars are suppressed if sub-clusters assemble after they
experience core collapse individually. In this case, sub-clusters form their
own multiple collision stars which experienced a few collisions, but they fail
to collide with each other after their host sub-clusters assemble. The multiple
collision stars scatter each other and escape, and furthermore the central
density of the remnant clusters had already been depleted for the stars to
experience more collisions. On the other hand, if sub-clusters assemble before
they experience core collapse, the multiple collisions of stars proceed
efficiently in the remnant cluster, and the collision products are more massive
than virialized solo-clusters and comparable in mass to cold solo-clusters.
