I have the following schema:

CREATE TABLE `user` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
);

CREATE TABLE `project` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`id`),
);

CREATE TABLE `project_conversation` (
  `project_id` int(10) unsigned NOT NULL,
  `user_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`project_id`,`user_id`)
)

I have ommitted the foreign key definitions for better readability. When I execute doctrine:mapping:convert (in Symfony2, but it seems it's a Doctrine2 issue), I get the following error:

[Doctrine\ORM\Mapping\MappingException]                                               
Property "user" in "Project" was already declared, but it must be declared only once

I have tracked down the issue to be caused by the existence of `user_id` in the project table. So basically, because `project_conversation` references `project` which in turn references `user`, `project_conversation` reference to `user` is perceived as duplicate.

I don't think that this should be the expected behavior though. user_id in `project` references the creator of the project while user_id in `project_conversation` references the creator of the conversation and thus I think the schema is valid.