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Probably because the rear assemblies don't turn, only move up and down.
The front assemblies turn w/ the steering, so there's the possibility of the collars turning and losing adjustment. The two collars basically work like a jam nut, tighten them against each other to lock in the setting.
The TEIN coilovers on my Toyota daily driver have double collars on all 4 corners. It just depends.
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