That's not supposed to happen...
Latest version of forge, no additional mods installed. Duped blocks can be interacted with to a degree. You can stand on them, and break them, and push them with pistons, but if you try to place a block on one the glitched block is replaced by the one you're placing.
From what I can tell, this same bug has appeared in snapshot versions of the vanilla game before. No Idea how forge reintroduced it, but it doesn't occur if I run the game vanilla.
My "latest" log, so I won't be banned:
Please help? This bug breaks the very reason I was running a modded client to begin with, and invalidates some of my recent life choices.