**EDIT**

Working as intended, cannot place slab of different type in same block space.

When attempting to place (in my example) copper slab, below an exposed cut copper slab, the block is not allowed to be placed, unless the ( upper slab block) exposed cut copper slab is made into copper slab.

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Survival World Example: https://1drv.ms/v/s!ArY94VX-LZYIj3MGcVcIF2levkBL?e=s6Zr7O

Creative World Example: https://1drv.ms/v/s!ArY94VX-LZYIj3TQhYFw5y3ATIgK?e=AHF38k