symdel implementation returns edges twice

[yutanagano]

If strings at index x and y are sufficiently similar to one another at distance d, the current symdel implementation returns both (x, y, d) and (y, x, d). Unless we plan to support non-symmetric divergences in place of proper metrics like levenshtein, this is redundant behaviour that can be harmful at large numbers of query strings. Is this behaviour intentional?