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Thank you for knocking on our door. Kiss the Mezuzah and step right in.
Q: Here’s a “headscratcher!” I can’t decide whether the mezuzah should go on the post to the right of a person going from my living room to my family room or on the right of one coming from the family room. I asked my rabbi whether I should put mezuzot on both posts to make sure that I have fulfilled the mitzvah according to all opinions. He told me that affixing two mezuzot was not a correct solution as it may be a violation of the Torah’s commandment: “Do not add on to the mitzvot.” I am confused because I know that many people put on two types of tefillin to fulfill that mitzvah according to two opinions. That’s just one example; there are many times that we do a mitzvah in two ways in doubtful situations. Why is mezuzah different?
A: As your rabbi said, the halachic custom is to refrain from affixing mezuzot on both posts in a doubtful situation as this may be considered “adding on” to the mitzvot. However, though this is the custom, your question is spot on, and many authorities have addressed it. In fact, some of them have asserted that there is no problem with affixing two mezuzot. Nevertheless, for whatever reason, the fact remains that many of our rabbis have reported that they have never heard that your solution was implemented.
If indeed there is no clear objective criteria to help you choose one post over the other, you may use your subjective judgement and place the mezuzah on the right post of a door you deem to be more logically an entrance rather than an exit. 
 Mezuzot Beitecha, Sh’ar HaTziun 289:45, citing Chazon Ish. Maharam Shik, cited in Binyan Tzion 100; Agur B’Ohalecha 15:4.
 Binyan Tzion 100.
 Daas Kedoshim 286:25; Shevet HaLevi 2:152:1; Minchas Yitzchak 1:9.
 Shevet HaLevi 2:152; Agur B’ohalecha 27:36. One should not recite a berachah on this placement. Cf. Igros Moshe Y.D. 1:176, who exempts this doorway completely.
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