Steps to reproduce:

1. Grab a sandstone slab and a purpur slab.
2. Place the sandstone slab down first and then place the purpur slab on top.
3. Notice that it turns into a double sandstone double half slab block.

This also works the other way around, and with placing the top-half slab first & the bottom-half last. It also occurs with stone & red sandstone slabs.

Cause of the bug:
This bug is caused by a mistake in the logic of the function that turns slabs into double slabs when right-clicked with another slab. The code only checks if the block is a "slab" and if it has the same metadata. It does NOT check for whether the block is stone_slab or stone_slab2. This is why this odd slab behavior occurs.