1/4 concrete board enough
Lots of people get particle board and OSB confused, please double check.
If it's particle board it should be removed, otherwise you're ok. Every backer board manufacturer says installing their board over 3/4" ply or OSB is just fine. Some even claim 5/8" is ok. I believe I've also seen 5/8 on 19.2 and 24" o.c. joists too.
The only time you have to have two layers of subfloor is if installing natural stone. Of course the more the better regardless whether it's necessary or not.
And yes, 1/4" is for floors, none of them add strength to the floor.
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