Versions
CraftBook version: 3.10.3 Build 4667
Paper build: 251

Describe the bug
You are able to dupe slabs with bridges.

To Reproduce

1. Create bridge with slabs one whole block tall (2x slab)
2. Break a part of bridge to receive 2 slabs
3. Replace broken part with 1 slab instead of two
4. Toggle bridge
5. Observe 2-slab high block replacing single slab

Expected behavior
You shouldn't be able to turn 1 slab into 2 slabs.

Fixed in 447c9f7

This is scheduled for being fixed in CraftBook5. Due to the way it works at the moment, more fundamental changes need to occur and the WorldEdit API will be used in CraftBook5 to overcome these issues.