STU02 OK, I've provided a fix for this issue. Version 0.3 introduced undo support for closed tabs and going on your description of Saft's undo support its basically providing the same functionality. That means when both plugins (SafariTabs and Saft) are installed then pressing cmd-Z (undo) to restore a tab will execute both SafariTabs and Saft's code resulting in 2 new tabs as you described. Its not really a fix but I've added a new option in the plugin's preference pane to enable/disable tab undo support depending on whether you have Saft insatlled.
I currently don't have Saft installed on my system but I'm pretty sure the fix will work. I've made the new version available on my site and not MacUpdate as of now until I'm sure the fix works. I'd appreciate it if you would try it out and let me know how it works.