At this moment you only can check if a relationset contains a relationship based on the relationship. This makes sense from an AT POV. As the AT would ask for a interesting relationship and then it will ask for all the targets.

But as bug 578602, for ATK implementation it would be also interesting to check if a relation-set contains a relationship with a specific target, to check if it is possible to add the same target twice.

Proposed API (final name is a work in progress):

/**
 * atk_relation_set_contains_extra:
 * @set: an #AtkRelationSet
 * @relationship: an #AtkRelationType
 * @target: an #AtkObject
 *
 * Determines whether the relation set contains a relation that matches the
 * specified type and target
 *
 * Returns: %TRUE if @relationship is the relationship type of a relation
 * in @set with @target as one of his targets, %FALSE otherwise

NOTE: this bug report will also solve the regression introduced on bug 578602 fix.