Does the Baco code require double 5/8" 0n the garage ceiling when there is a living space above ?
In residential construction up through duplexes, 1-hour Fire rating (1 layer) is required on the garage side of the ceiling and walls when there is a living space on the other side.
When you have 3 units or more, then you MAY need 2 layers, depending on the size of the complex and type of construction.
For ceilings in 1 and 2 unit residential units: 1 layer installed on 2x lumber and/or I-joists correctly gives you “1-hour” construction. In order to obtain the “1-hour” designation 5/8” type-X gypsum board must be installed on joists at 24” on center (max.) fastened with Type W or S drywall screws at 12” on center along all supports, and then “rough” taped. (See ICC Table 721.1(3), item 21.)
I have never seen local code in my life requiring a double sheet of dry or more than 5/8". Period.
To second this due to the weight of the 5/8" drywall there would be some innate issues installing it over the top of another 5/8" sheet. It could be done but your screws not having framing for the first 5/8" is a recipe for failure. So either this is the weirdest city in America or you are reading code wrong.
Note: It is very normal to require 5/8" drywall on all adjoining walls. Which means your ceiling would need to abide.