Good post, lack of memory on the civ level is a real problem. Local solution is to read more of others, and be less "original." Imo, LW could stand to move more in that direction.
The story of etiology of scurvy is a sad one also (and that was much shorter time frames and in recent times).
edit : Graphical models must have been rediscovered 4 or 5 times in the 20th century. Info theory people recently "rediscovered" the g-formula: http://infostructuralist.wordpress.com/2010/09/23/directed-stochastic-kernels-and-causal-interventions/
etc, etc.
On the other hand, discovering something for yourself is arguably one of the best ways to truly understand it, and it does not require you to read & memorize lots of sundry information for the occasional thing that actually ends up being useful.
Related: Son of Low Hanging Fruit, Low Hanging Poop
A post by Gregory Cochran's and Henry Harpending's blog West Hunter.