Double counting baseline calories while logging exercise

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Suppose one has BMR of 2400. Ie the person burns 100kcal an hour by just existing.

If that person walks for an hour as cardio and logs it as cardio in exercise diary or gets the kcal burnt from a watch or an app - let's say 400 kcal.

Does this 400 include the 100 which is going to be burnt anyway? Ie does one earn 400 more kcal or only 300 more?

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