how far away is the nearest cell that matches this one?

Insanely tight! Hats off to you XOR LX, hope it's a great day for you there in Yorkshire, England.

Thanks a million!! Tweaked it so scan column C and D instead of just A1:A6 (entered as an array formula just as you described, by hitting Ctrl-Shift-Enter there after the final closing paren):

=MIN(IF(C:D=C1,IF(ROW(C:D)<>ROWS($1:1),ABS(ROW(C:D)-ROWS($1:1)))))