Progressive overload question

Sort of.

Think of training load as duration x intensity.  In your example, you're applying roughly the same training load in both workouts (total of 30 minutes at 85%), assuming that you haven't reset the threshold that you're basing the 85% on.  I'm putting that caveat on my answer, because if you're talking about training with power, and between the sessions you retest your Functional Threshold Power (FTP), find it increased, then do the next session at 85% of the new FTP, then you're applying a greater training stress (load) in the second session, so you've progressed.  The other factors that come into play are fatigue, environmental conditions, and life stresses.  If you're more fatigued, it's hot & humid, or you're stressed by work or family issues, the session can be more stressful than if these influences weren't present.

In any event, repeating the same sessions over and over quickly leads to stagnation, even if you're adjusting them slightly.  Instead, only repeat a session a couple times over a period of a few weeks, then adjust your program.  This has the dual benefit of achieving a greater rate of improvements and avoiding boredom.

It's adding more stress by increasing duration and/or increasing the output. What you continually add the "more" to depends on what aspect of fitness you want to stress.