When attempting to use a sleeping bag when a mod that prevents sleeping is installed, (Iguana Tweaks in this case) The sleeping bag doesn't allow the player to sleep (like intended by Iguana Tweakks) but it duplicates itself every time the player tries to sleep.

Are you sure it's Iguana Tweaks? I don't see any option like that

It's just iguana tweaks. Not iguana tinkers tweaks. And the option is near the bottom of the config file at config/iguana tweaks/main.cfg.

And sleeping is disabled by default I believe.

Well, I installed this, made sure disableSleeping=true, but I can't reproduce it. Normal chestpiece blinks in sleeping bag slot (expected) but I still end with just single copy chestpiece and sleeping bag.

EDIT: are you sure you aren't in creative mode?

I was in creative when I was testing to see if the sleeping bag would cheat around the no sleeping. But I was right clicking with the sleeping bag in my hand instead of equipping it in my inventory if that makes a difference.

No. Item will always be duplicated in creative, it's vanilla stuff.

Then in that case… oops. Well thanks for clearing that up. I need to remember to do my testing in the proper game mode in the future.

What version are you using?

1.4.3b
Should've known I needed to include that info