This is a cumulative achievement but the same secrets do not count towards your total. In order to get this achievement, you'll have to find 70 separate secrets. While this is not all of them, this is a fair number of them. If you need help, 3D Realms has a legacy walkthrough that shows you the locations of all the secrets in game available at
http://legacy.3drealms.com/duke3d/walkthrough/. You can open that link in your Steam overlay as you play the game and follow along if you want. Keep in mind that this website does not cover the brand new episode but if you find all the secrets through the first four episodes, you will not need to find any in that episode.
Along the way, you'll also get the All Too Easy achievement for finding 1 secret.