“Strong” and “strength” are just slightly different versions of the same Greek word. Perhaps you have also noticed that this Greek word is very similar to some English words; dynamo, dynamite, dynamic, and dynamism, for instance. All these are words that speak of great power, and this is obviously the idea here, repeated for emphasis.
It’s kind of fun to play with the English equivalents in paraphrasing Paul’s prayer: that God would make us “spiritual dynamite”. Maybe you remember an old ACDC song that went, I’m T.N.T , I’m dynamite, T.N.T, and I'll win the fight? That’s not far from what Paul is praying here, so long as we remember that it is God’s might and not our own that makes us powerful, and that our fight is not with flesh and blood but with “all the spiritual forces which corrupt and destroy the creatures of God.” Are we T.N.T?
Or how about thinking of this prayers as asking that God would make us “dynamized dynamos”? Just how dynamic are we as Christians? Would anyone describe us as a real dynamo for God’s kingdom?
God’s power, the very same power that raised Jesus from the dead, is available to each and every one of us. How is that power being manifest in your life and in mine?