Give more penalty on assigning matches with same players
When two lowest-rating (or estimated rating) players happened to make a match, there was a high chance that they would be paired on the next match again.
There was a report on this issue: http://bleu48.hatenablog.com/entry/2018/04/05/125107. The scenario would be
APTkai_i9_7960X and kifuwarabe2018 ended up with playing 12 successive games from 2018-04-06 11:00 to 2018-04-06 16:30.
As a quick workaround, raise the same-pair penalty.
Closed at e40247f1908aadad6ed15e89a3b6bde935960095