On my stock 6.0.1 Verizon Wireless Note 4 I have had an extra beep after every notification sound for several months, but it never bothered me enough to try to search for a solution until tonight. I found this: https://androidforums.com/threads/extra-beep-after-each-notifications.1119027/ which described my problem perfectly, and gave a solution involving turning off the "new normalization method" setting in the Disable Increasing Ring app. This change resolved my problem, so I thought I should describe the problem and solution here in case this affected anyone else. I have reported the problem to the app developer.
Larry
Larry