Duvessa is the anglicized form of Dubh Essa meaning “black waterfall”, made up of Irish dubh (black; black-haired) and eas (waterfall, cascade, rapid). Dubh Essa (also spelled Dubhessa) was a fairy common given name in medieval Ireland, in the 13th/14th century.
Duvessa was used in Irish playwright M.J. Molloy’s play The Wooing of Duvessa (1964).
Douglas is the anglicized form of Scottish surname Dubhghlas meaning “dark river” or “dark stream”, composed of Scottish Gaelic dubh (black) and glais (water; green). It comes from the name of a river in Scotland from which the name of the clan Douglas derives its name from.
Interestingly, Douglas was originally used as a unisex name in the early 17th and 18th century but eventually it only became used as a male name.