I'm assuming this was unintended, as receiving both is very nonsensical.
Instead of having a 75% chance to get a module casing and a 25% percent chance to get a T1 module, there is the possibility to get none or both. On large scale this doesn't mean anything, because the distribution is the same, but when crafting your first few modules by hand, it's very weird that the amount of modules you get from x module casings can vary by one or two.
I'm assuming this is due to the way that Factorio interprets percentages in recipe outputs.
I haven't unlocked T2 and T3 modules yet, but I assume these have the exact same problem.

That is how random recipes work in the engine and there isn't a way around it (fingers crossed for 2.0)

Ah, that makes sense. Thanks for clearing it up.