Duo is a brand by Cisco, providing two-factor authentication, endpoint remediation, and secure single sign-on tools. The logo uses caps from Baro Black, featuring the alternate O with horizontal bridge and a customized D that follows suit. It’s combined with typography in Neue Haas Grotesk. From Duo’s brand book:
The name Duo is a reference to our roots in two-factor authentication; our two co-founders, Dug and Jon; and the duality of our industry – approve vs deny, good vs evil, white hat vs black hat. The logo is a visual analogy of a lockset tumbler mechanism, with the middle “U” being closed, to symbolize securing the lock.