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Hi. Could we use the word "beloved" to address someone in a letter? Thank you for your help in advance.

eg,
Dear beloved friends
Dear beloved John Doe
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Generally we don't include beloved when addressing someone in a letter. I could imagine Dear beloved friends being used in rare circumstances, perhaps by a preacher writing to his congregation. Dear beloved John Doe sounds very odd to me. I would be surprised to see it used.
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'Beloved' is like an extreme form of 'dear', so using both together is redundant.

Dear friends normal and common

Beloved friends Uncommon, except eg when a priest is speaking in Church.

Clive