Tosfos [Shabbos 104b] says that if one writes on top of an already existing כתב it is not considered כתב and he is פטור on Shabbos. However, if the second כתב ADDS to the first כתב like a גט where the first כתב was written שלא לשמה and the second כתב was לשמה, then the second כתב is considered כתב and one is חייב on Shabbos.
The Minchas Chinuch [מצוה ל"ב אות ל"ד] says that this makes sense if he writes בשוגג but what about when he writes במזיד? If the second כתב is valid then the writer is a רשע because he was מחלל שבת במזיד and then the גט is invalid because he has a דין of a גוי who can't write לשמה. Now that the גט is invalid it turns out that he has not been מחלל שבת because כתב על גבי כתב is not considered writing. If so the גט IS valid. If the גט is valid then it turns out that he WAS מחלל שבת which would in turn make the גט invalid. Invalid? He is not a מחלל שבת and the גט is kosher. Kosher גט? He is a מחלל שבת ...
AD INFINITUM!! [I am a yeshivishe guy who likes to show off my knowledge of Latin].
