Peter

This is a fix for https://github.com/Masterminds/squirrel/issues/281

Calling `query.Distinct().Distinct()` results in the `DISTINCT` option being applied twice, which is invalid SQL. See the playground snippet here: https://play.golang.org/p/9qwFGj1jRWx ```golang builder := sq.StatementBuilder.PlaceholderFormat(sq.Dollar) q1 := builder.Select("foo").From("bar").Distinct() sql, _, _...