You certainly could do some more strength training, but in terms of duathlon you would be better served by splitting that extra time between bike and run. Probably leaning more towards run

You can for sure sub in some lifting, but I would consider re-allocating some of that training time to the run and or bike... Not too familiar with that plan, but if it called for two swim sessions per week of 1hr ish each, I would consider two 30min strength sessions, and using the other hour for a run/bike brick. I mainly race Du, and I often do something similar to what I described when I finish my last tri for the year and only have Duathlons left...

I would turn my attention to yoga/stretching during those swim sessions. DU training is hard enough without the break on the body that swimming affords you.
Good luck, John

That's kind of what I was thinking. Couple of basic lifts weekly and few additional bike/run workouts at appropriate intensities.

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Human Person

Yes to lifting. bike/run makes you very skinny. Swim/weights keeps you healthy.