Using the 5.2 API, it is possible to assign the same role with the same subtree limitations repeatedly.

Setup:
1. Create a user group
2. Via the API, call the following:

$group = $userService->loadUserGroup( <group ID> );
$profileRole = $roleService->loadRoleByIdentifier( 'some_role' );
$limitation = new SubtreeLimitation( array( 'limitationValues' => array( '/1/2/214/' ) ) );
$roleService->assignRoleToUserGroup( $profileRole, $group, $limitation );
// Second call
$roleService->assignRoleToUserGroup( $profileRole, $group, $limitation );

Expected:
Looking at the user group in the admin interface, you should only see the single role

Actual:
You will see the role and limitation applied twice. Note: this does not occur when applying roles via the admin interface.