One join table for two Many-to-Many relations - symfony

Question

I'm working on a solution for adding tags to two differente entities.
in order to get data easily in the frontend i created a joinTable named Tag_Mapping like this :
class TagMapping
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\ManyToOne(targetEntity="Tag", inversedBy="tags")
*/
private $tag;
/**
* #ORM\ManyToOne(targetEntity="Feed", inversedBy="tags")
*/
private $feed;
/**
* #ORM\ManyToOne(targetEntity="Question", inversedBy="tags")
*/
private $question;
...
}
The Tag Entity :
class Tag
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255, unique=true)
*/
private $name;
/**
* #Gedmo\Slug(fields={"name"})
* #ORM\Column(unique=true)
*/
private $slug;
/**
* #ORM\OneToMany(targetEntity="TagMapping", mappedBy="tag", cascade="remove")
*/
private $tags;
...
}
and in both other entities (Feed and Question) I made reference to TagMapping entity like this
...
/**
* #ORM\ManyToMany(targetEntity="Tag")
* #JoinTable(name="tag_mapping",
* joinColumns={#JoinColumn(name="feed_id", referencedColumnName="id")},
* inverseJoinColumns={#JoinColumn(name="tag_id", referencedColumnName="id")}
* )
*/
private $tags;
...
the problem I'm facing is that it's not a valid way to do it, as it shows an error when i'm trying to execute :
php app/console doctrine:schema:update --force
saying that tag_mapping table already exists.
do you have any idea how can i get it done using only one joinTable instead of one for each relation ?
Thanks.