Strange portal reaction after Validation

[stevie.at]

I'm working on a multi-file db with several portals in different files. To avoid duplicate entries I use a calculation field set to unique as validation. This seems to work pretty well in every single portal except these ones related to a file with all the clients' data. In this case, afer pushing the revert-button the validation error-message provides, the portal seems not to refresh again and the sort-order is turned upside down...

The protal is originally sorted by last names. If you try enter the same record twice you'll get the error message but afterwards the portal appears sortet in the order of creation.

All portals have the same setup, also all relationships should be the same. I also got the FM7 update installed...does anyone know?