Question: We recently bought a new house and are affixing our mezuzos. There are some old mezuzos on the doorposts which have been painted over multiple times and are likely not kosher. Instead of removing them, can we just affix our new ones next to them?
Answer: The Sifrei (Devarim 82) writes that one who adds on a fifth corner to one’s tzitzis or a fifth species to the arba minim has transgressed bal tosif, the prohibition against adding to the Torah. Likewise, Tosafos (Rosh Hashana 28b) writes that if one added a fifth parsha to a pair of tefillin, they would have transgressed this prohibition.
R’ Avraham Dovid Wahrman (Daas Kedoshim 286:25) applies this to the mitzva of mezuza, too, writing that one mustn’t affix multiple mezuzos to one doorpost (See Minchas Yitzchak 1:9).
The Pischei Teshuva (YD 291:2) quotes the Chamudei Daniel (YD 291:1) who adds that one must remove the old mezuza before affixing a second, and one must be meticulous not to ruin the old mezuza when removing it.
R’ Moshe Feinstein (YD 1:176; YD 1:183) adds that one must remove the old mezuzos even if they have been repainted over multiple times and are likely passul, and even though by doing so, one may scratch the doorframe (See Shevet Halevi 8:169).
In conclusion, one must remove any old mezuzos before affixing new ones, even if one believes that they may not be kosher.