Is it common for someone with a substance abuse illness to also develop dual diagnosis?

It's just that I have this friend who has always been an alcoholic. Lately, I have noticed that he has gotten more depressed as well. I was just thinking that maybe he has dual diagnosis.

: http://alcoholdrugstreatment.info/is-it-common-for-someone-with-a-substance-abuse-illness-to-also-develop-dual-diagnosis