The use of the term rebuilt
(see on Nehemiah 3:2) suggests that the Babylonians had so thoroughly destroyed this gate that it needed to be rebuilt from scratch. Once the team had retrieved and rebuilt the stone structure of the gate, they could install the doors with their hardware.
14 Malchijah the son of Rechab, ruler of the district of Beth-haccherem, repaired the Dung Gate. He rebuilt it and set its doors, its bolts, and its bars.