(The). The cathedral.
“The supreme executive of Florence suspended Savonarola from
preaching in the Duomo.” —
Dup is do up. Thus Ophelia says, in one of her snatches, he “dupt the chamber door,” i.e. did up or pushed up the latch, in order to open the door, that he might “let in the maid” (Hamlet, iv. 1). A portcullis and some other doors were lifted up or dupped.
“Iche weene the porters are drunk. Will they not dup the gate to-day.” —Edwards: Damon and Pithias (1571).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Duomo from Infoplease: