We do think about them, and they seldom occur, and if they do occur, nothing terrible happens. I bet if we took a bunch of average duplex 5-15's off people's walls and loaded them up to 2x15A, and the same for a bunch of double 13's loaded to 2x13A, we would see as much excess heat from the 5-15's as from the 13's.
About the only time I have ever seen two 13A loads regularly plugged into one double socket was back in the 1970s in my dad's shop. There was a ducted warm air heating system but it wasn't really powerful enough for a cold winter and the vents were in the wrong places, so they tended to supplement it with some portable heaters. I remember two 3kW fanheaters used to sit side by side plugged into a double socket in the main sales floor. It lasted all the years I knew the shop, and that is about as exciting as 26A on a double 13A usually gets.
I suppose my main point, as I tried to convey with my priority list of heat sources, is that BS1363 is a safe and reliable connector, although it does tend to heat more that some others due to the presence of the fuse and its contacts mainly if they are of cheap manufacture. Thermal failures due to total loads on doubles in the 20-26A range are utterly insignificant compared to thermal failures due to loads in the 10-13A range on one plug, which are not exactly common either.
