QueryBuilder allows joining with the same alias multiple times

Currently, QueryBuilder allows assembling broken DQL like

SELECT u FROM User u JOIN u.posts p JOIN u.logs p

or

SELECT u FROM User u JOIN u.posts u

It may be interesting if we either replace DQL parts by associating them to their alias.

Another solution, since Alexander noted that it is a dumb string builder, is to throw exceptions early to disallow broken DQL generation (such as in the second case).

It may also be useful to allow giving insights about the currently existing joined aliases.

public function hasJoinedAlias($alias);