What is the difference in nuance of ~긴 if I say 날씨 보니까 진짜 점점 가을이 오긴 오나 봐 instead of 가을이 오나 봐?

See a translation

-긴 하다 implies the speaker's reluctant acceptance of something that the speaker is mentioning, while the speaker admits that there's undeniable and unavoidable truth about it. Its like to express a little uncertainty about a thing like "I really don't like to admit the fact, but I can't deny this because what I see is reality”. So,  -긴 하나 봐 means "Although I'm not 100% sure of it, it seems true that --". 
가을이 오긴 오나 봐 is the same as 가을이 오긴 하나 봐. It means "Although I'm not 100% sure of it, it seems true that fall is coming." 가을이 오나 봐 is simply "Fall seems to be coming".