  Planet-planet collisions are a common outcome of instability in systems of
transiting planets close to the star, as well as occurring during in-situ
formation of such planets from embryos. Previous N-body studies of instability
amongst transiting planets have assumed that collisions result in perfect
merging. Here, we explore the effects of implementing a more realistic
collision prescription on the outcomes of instability and in-situ formation at
orbital radii of a few tenths of an au. There is a strong effect on the outcome
of the growth of planetary embryos, so long as the debris thrown off in
collisions is rapidly removed from the system (which happens by collisional
processing to dust, and then removal by radiation forces) and embryos are small
($< 0.1\mathrm{\,M}_\oplus$). If this is the case, then systems form fewer
detectable ($\gtrsim1\mathrm{\,M}_\oplus$) planets than systems evolved under
the assumption of perfect merging in collisions. This provides some
contribution to the "Kepler Dichotomy": the observed over-abundance of
single-planet systems. The effects of changing the collision prescription on
unstable mature systems of super-Earths are less pronounced. Perfect mergers
only account for a minority of collision outcomes in such systems, but most
collisions resulting in mass loss are grazing impacts in which only a few per
cent. of mass is lost. As a result, there is little impact on the final masses
and multiplicities of the systems after instability when compared to systems
evolved under the assumption that collisions always result in perfect merging.
