
Samsung GALAXY S4 DUOS
The latest firmware for Samsung GALAXY S4 DUOS (GT-I9502) is I9502ZNUJOH2 with Android 5.0.1 (Jelly), released on August 18, 2015 for 1 region (CSC). Every official firmware build for GT-I9502, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/I9502ZNUJOH2
GT-I9502 started on Android 4 and has received 1 major Android upgrade; it now runs Android 5.0.1 (Jelly). The newest build, I9502ZNUJOH2, was released on August 18, 2015. No new build has appeared for 136 months, so regular updates for this model have most likely ended.

GT-I9308

GT-I9268

GT-I9228

GT-I9210

GT-I9108
The newest firmware for GT-I9502 runs Android 5.0.1 (Jelly): build I9502ZNUJOH2, released on August 18, 2015.
The latest build for GT-I9502, I9502ZNUJOH2, was released on August 18, 2015 (11 years ago).
GT-I9502 has had no new firmware for 136 months, so regular updates have most likely ended.
Yes, all firmware files for GT-I9502 are 100% official, original, and obtained directly from Samsung servers.
To flash GT-I9502, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing GT-I9502.