In the February 1973 Word Ways, Darryl Francis searched for examples of words having two doubled letters (as abbess and buccaneer). Specifically, he attempted to find examples for all doubled pairs excluding J, Q, V and X -- that is, words containing aa-aa, aa-bb, aa-cc, ... , yy-zz, zz-zz.
Cohen, Philip M.
"Words With Two Doubled Letters,"
Word Ways: Vol. 10
, Article 19.
Available at: https://digitalcommons.butler.edu/wordways/vol10/iss4/19