Just to throw in there. Standard would be a single 2x (or 3x) plate with 2 top plates with. PT over concrete and such.

Only thing I can think of with doubling plates.

Haven't built anything with doubled sill on bottom but have built with 3x bottom plate and a few 3x studs at breaks for shear.

On second floor apps, even with doubled plate over certain types of joist spans (?? as mentioned) would leave me with having to attach plates with SDS screws or some hardware, that wouldn't be any fun.

So, in short doubled joists are common as are double top plates.

Tim

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