mining duplicates the gas next to it

I can't reproduce this.  While digging a block as a test, right before the block broke, there were 1640g of oxygen in the adjacent area.  right after the block broke, there were 1440g (and falling) in the adjacent area and 200g (and rising) in the newly cleared area.  Digging more blocks and carefully observing the gas gives similar results.

I took a look at this. It appears to me that mining a tile creates a vacuum, as it should. If you pause the game the split second after a tile is mined and check the gas in that spot, you will see it is very low pressure. Also remember that when dupes are standing in one spot mining, they're breathing out CO2, which when you have 3 or 4 dupes standing together will create a big pocket of CO2.