Glider/glider collisions were very few and far between in the 1970s, increased in frequency in the following decades, and in recent years have been slightly fewer. We don’t know if the latter is a genuine improvement or a statistical blip.
For about three decades, they accounted for an average of nearly one fatality a year, or nearly a third of total gliding fatalities.
There are also non-fatal collisions, of course. It is estimated that about half of those which could have been fatal, in fact were saved from being so by pilots having and using parachutes.
In the UK, it is not a legal requirement to wear a parachute in a glider. Most if not all gliding clubs insist on all pilots wearing parachutes in club gliders, however, unless the pilot is simply too heavy and has to take a cushion so as still to fit the recess in the seat. Similarly, almost all privately owned gliders are flown by pilots wearing parachutes.
Overall accident rates in gliders are rather better than power flying, AIUI, but the relative incidence of causes is quite different. For example, CFIT (controlled flight into terrain) is a relatively common cause of fatal accidents in power flying but almost unknown in gliding.
Learning gliding is pretty safe, as is learning power flying. The dangers seem to come later, when one form or another of pilot error is the most common factor - and that is up to us.
Chris N