GT-I9082 - Unable to enter download mode through ADB
Luckily I have Grand Duos too and I can confirm it is perfectly bootable into Odin Mode (aka Download) by

adb reboot download


Tried it just now.

What I can propose to you:


Try to get rid of all other emulated/physical devices by restarting ADB-server and/or ejecting non-necessary ones.

adb kill-server



I can guess you have them as you use -s clause.


Learn the difference between Odin Mode, recovery mode and bootloader mode. Maybe you are confusing them?


I have custom recovery but this doesn't prevent me from booting into all this modes.