PF_Cmd_USER_CHANGED_PARAM called twice on button press in Premiere

This is a hack workaround, as opposed to figuring out why the event is triggering twice, but this might work.

In your User Changed Param callback, have a static A_short variable initialized to 0. Check for this being non-zero to open your file dialog, in other words ignore the first button hit and act on the second.

static PF_Err UserChangedParam(...) {

    static A_short button_hit_count = 0;

    if(param == your_button) {

          if(button_hit_count == 0) {

              // Ignore the first hit

              button_hit_count++;

              return PF_Err_NONE;

          }

          // As button_hit_count is now 1, Open file dialog

          ...

          // Reset button hit count

          button_hit_count = 0;

    }

    return PF_Err_NONE;

}