With Double Slabs installed I've found I have to double right-click to place any non-slab blocks while crouched, and also to pick up entities using the Carry-On mod. Sometimes even more than just double-clicking. This also applies to any other block interaction like hoe-ing, sowing, creating paths, and similar.

I looked into it more and it seems to be a soft compatibility issue with Just Enough Keys, as it has a odd interaction with it's expanded hotkey functions.