Question in an Agatha Christie article
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I am currently reading a newspaper article by Agatha Christie. The article was included in Detection Club book SIX AGAINST THE YARD and was about a serial poisoning case. And in the article Agatha Christie mentioned a name "Mary Ann". The name has no relationship to the murder case and I was wondering whether it is some kind of idiom? I attach the paragraph in the pictures below. Thank you very much for your help.
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