Is there anything special one needs to do when doubling a bread recipe?
Does one just use twice as much of everthing?

I am a novice. After about three tries, and some adjustments, I finally
settled on a multi-grain bread recipe that I (and my family) like. It
ends up with the texture of "regular" sandwich bread. But, my recipe
only makes two loaves.

Yesterday, I doubled the recipe and instead of ending up with 4 clones
of the nice loaves I had made before, I ended up with 4 pretty dense,
semi-risen loaves. Edible but heavy.

Here's my basic process: I mix the dough, let it rest for 20 minutes,
add the salt & knead it, let it rise, shape it into loaves, let the
loaves rise, and then bake. I use rapid rise yeast.

The only thing I noticed this time around that struck me as different is
that when I "rested" the dough, the dough had noticeably risen. I am
sure I let it rest around the same amount of time. I'm not saying that
this is the only thing that was actually different, I am pretty new to
this, so I may simply overlooked some key thing.

Any ideas about what I goofed up?