Still confused about double items

[kayatoast]

Does double lump of coal stack?
Double moms knife,proptosis and brimstone...is it just double damage or nothing
Does doubling the same guppy item count as two guppy items?
pls help

[Kares]

I think it works just for stat up items,also idk about guppy,but in rebirth having 2x 9 lives and other guppy item didnt turn into guppy

[dispensing product]

I think a lot do. In fact about two runs ago I managed to pick up two of the 50% steam sales from fighting greed. After that all items in all the stores I came across after that were free.

[unendingEmpathy]

You know what, I wanna test this theory with double Cricket Heads because if I can use that double-item item with a car battery, I'm pretty sure Isaac would go from 2 to 50 in a second.