What you appear to have is a document with automatic paragraph 'list-numbering', using incrementing characters (i.e. a, b, c, d, ...). Word's mailmerge often doesn't work well with those, as the sequence tends to simply continue rather than re-starting for each letter. The simplest solution is to not use the automatic paragraph 'list-numbering' and, instead, number the paragraphs manually.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]