One way would be to convert the document to a table, with rows separated by the paragraph breaks, then sort the table. You could then convert the table back to text. Sorted this way, you will have all the paragraphs that start the same grouped together until the first character difference. Do note, though, there is a limit to how many characters Word will use for a table sort (~32 IIRC).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]