i came across the word "resiliency" in a sports column and wondered: "What happened to resilience?" which means the same? Then it occurred to me there is a group of abstract nouns formed from adjectives that follow the same pattern. Constance/constancy. Hesitance/hesitancy. Dependence/dependency. And so on. (But not independence/independency.) What is going on here? Why the duplication?

Maybe its just personal choice....

Good observation though.

But they're not always interchangeable:

"His excellence and constancy are an example to us all."
"His Excellency and Constance are an example to us all."

Then there's emergence/emergency...

I hope you're not expecting consistency from language, nor consistence neither.