What is the measure for scaling cinnamon if you double a recipe?
If you never prepared the recipe as written (which appears to be the case), you have no basis to tie the way it tastes when doubled to doubling it, rather than to the proportions of the original recipe. 

Based on many years of making many things in many sized batches, if I double a recipe and want it to taste the same as the original recipe, I double the cinnamon. 

I conclude that the most likely case here is that the original recipe had a lot of cinnamon, and that what you tasted is simply the way it tastes. 

Depending what travels the recipe might have had before you met it, there's also a not-uncommon error that occurs with US measurements sometimes - the confusing (by some transcriber in the past) of T and t in hyper-abbreviated notation, which can make a factor of 3 difference (T = Tbs =Tablespoon = 15 ml, t = tsp = teaspoon = 5 ml.) The copy of the recipe you are looking at need not be hyper-abbreviated for this to have happened to it at some point in its past