The Jewish Household

I am unsure to which doorpost to affix a mezuza. May I affix one on both sides, or is that prohibited as bal tosif (adding on to a mitzva)?

A mistake was made in filling out my ketuba and the identical copy for the certifying beit din. We had a spare ketuba but not an additional one to use as a copy. Now, several days after my wedding, can I ask the witnesses to sign a copy with the same information, including the date of the wedding? (The mesader kiddushin and witnesses are talmidei chachamim but not poskim, and they encouraged me to ask this shayla.)

Sarah relinquished her rights (mechila) to her ketuba in order to encourage her husband, Reuven, to give her a get. Now she wants to back out of this step. Perhaps we should say that simple oral mechila without a kinyan is not effective when the creditor (i.e., the wife) still has a document (i.e., the ketuba) that states that the debtor owes her money.

Is there timtum halev [approximately, spiritual pollution of the heart] when one ingests non-kosher food in a halachically valid manner, e.g., based on bitul (nullification)?

Reuven sold supposedly kosher wine on credit to Shimon, who had a retail business. Shimon sold it to several customers. Later on, it became clear based on witnesses and Reuven’s admission that it was stam yeinam (wine handled by non-Jews, which is Rabbinically forbidden). To what extent does Shimon have to pay Reuven for the wine that Shimon already sold? What should be the arrangement be between Shimon and the people whom he accidentally caused to drink non-kosher wine?