Minhag oker Halacha has a restriction, there has to be a Maan Deamar that agrees with it. Even a single opinion. This is because normally we go after the majority opinion but when there is an established Minhag like a minority we can/should side that way. (however in monetary situations it is not like this, there you do not need an existing opinion. Thats just how Choshen Mishpat works)
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