validation called twice when using related_subform

When using related_subform along with Relativity, it seems that drupal_validate_form() gets called twice on the same form (or subform?) when editing and saving an existing parent node that has a related child. This results in an error message "This content has been modified by another user, changes cannot be saved", as reported at http://drupal.org/node/278341, and then the node cannot be saved.

I don't understand enough about how all these modules are working together, and this is probably a very bad solution, but I found that when I made the change in the attached patch, I could avoid this error. I hope this helps someone knowledgeable enough to find the actual problem.