I agree with our friend from NY about the backerboard, (or membrane), but although the more the better, there is no requirement by any manufacturer or tile association that says you need 1" of plywood. Matter of fact all say 3/4" ply or OSB is plenty and many say even 5/8" ply is good enough on 16" oc joists and properly installed of course as in all cases.
I do not like a single 5/8" ply, but 3/4" I've done hundreds of times and as noted more than meets standards.
TILE GUY - retired- TROY, MI - Method & Product suitability consulting.
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