Duo Security vs Google reCaptcha: What are the differences?

Developers describe Duo Security as "Verifies the identity of your users with two-factor authentication". In order to trust that your users are who they say they are, we verify their identity with an easy-to-use two-factor authentication solution, while giving you the ability to enforce stronger user access policies. On the other hand, Google reCaptcha is detailed as "A free service that protects your website from spam and abuse". It is a free service that protects your website from spam and abuse. It uses an advanced risk analysis engine and adaptive CAPTCHAs to keep automated software from engaging in abusive activities on your site. It does this while letting your valid users pass through with ease.

Duo Security and Google reCaptcha can be primarily classified as "Security" tools.