up-to-date with leela-zero/next (2019-02-22).
The reason for duplicate games is cached NN eval results. Two approaches to eliminate these:
clear_nncache.exe clears the cache each time clear_board or loadsgf is called.
remove_min_cache_count.exe removes the lower bound on the size of the cache, so the cache size won't be set to a high number (6,000) if playouts/visits is low, so when a new game is started, the cached positions from the start of the last game won't persist.