CiviCase assign new follow up bd error : already exists

I was trying to assign a new follow up in a case to somebody else - so it was open the case - new activity follow up then in the "assign to" box add an individual and I get a

DB Error: already exists error
CiviCRM version:
4.31
CMS version:
Drupal 7.22
MySQL version:
5.5.31
PHP version:
3.4.10.1deb1
I would like to confirm that error shows in "out of the box" civicase with standard "housing support' casetype.

Forum link: http://forum.civicrm.org/index.php?topic=28502.new;topicseen#new