Duke, 33 has previously been on the books at Bramall Lane when he signed from Alfreton Town in September 1999 under the then leadership of Adrian Heath and left twelve months later to join Burton Albion who were then out of the Football League.
He did spend a brief period on loan at Bury whilst with the Blades but like his united career failed to make a first team appearance.
Hull City signed the keeper for £20,000 in July 2004 and has since also featured for Stockport County, Wycombe Wanderers and recently Derby County all on loan.
Duke is expected to sign for the Blades in a low figure sum in the next few following weeks.