This is implementation of multiple keys in index for single record. I guess it can be useful for making full-text search indexes or similar. Yes, keys collision is not very good and slow down insertion. But queries which return many entries for text searching is also useless. So may be it is possible to create such indexes with reasonable hash collisions (for example search only for words with which length is greater than 4 chars etc...).
Thank you for this pull request BUT i have to decline it because it will cause slowdown for non multiple indexes. Because it's the second pull request about that feature and the second where we can't accept it we will push our MultipleIndex solution right now. That solution uses "index power" to archive exactly that without overhead on non multiple indexes. Expect that commit in hour or so.
Thank very much for your approach to solve it. But you forgot about delete / update operations.