Based on this topic:
There is a duplicate line in one of the files in /etc/opkg/, either in customfeeds.conf or distfeeds.conf.
I'd move to that folder with: cd /etc/opkg, and then look at the content of each file with:
less customfeeds.conf
less distfeeds.conf
One of those files will contain a duplicated entry with the text: openwrt_kmods. Possibly the entire line you see in the error message. Try to find which file it is, and I can try to guide you on removing the duplicate line.
Sorry for being so vague, I'm working blind, as I don't have the same file contents you do.