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Are they the same thing? Or slightly different?

Never came across the term avoidability in my K's class. However, a quick Google search indicates to me that the two terms represent the same concept.

I took them to mean about the same thing. Either one you are trying to lessen the amount of damages you attain on account of the other parties breach. However, it doesn't have to be the best decision just a reasonable one, so you just need to show you made an effort to mitigate the damages under the circumstances.