You really should be using the same tags regardless of the document. After all, that's how the master list in Word works - which you should have checked out before imposing your own schema. It seems to me you now need to fix your tags to produce something more consistent with: (a) how Word works; and (b) your goals (i.e. using the same tag for a given reference across all documents).

When Word creates the tags, they're usually along the lines of the first three letters of the Author's surname plus the last two digits of the year of publication. Naturally, this varies where the same surname appears more than once in a given year (e.g. by the addition of an extra digit).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]