Albert Barnes Commentary Psalms 62:11

"God hath spoken once, Twice have I heard this, That power belongeth unto God." — Psalms 62:11 (ASV)

God has spoken once; twice have I heard this - This repetition, or this declaration that he had heard the thing repeated, is designed to give emphasis to what was said, or to call attention to it as particularly worthy of notice. See the notes on Job 33:14. Compare Job 40:5. The sentiment here is particularly important, or is deserving of special attention, because, as the psalmist had shown, all other resources fail, and confidence is to be placed in nothing else for what humanity so greatly needs; neither in people, whether of low or high status (Psalms 62:9); not in oppressive acts—acts of mere power; not in plunder; not in wealth, however acquired (Psalms 62:10).

That power belongs to God - (Margin: strength). The idea is that the strength a person needs—the ability to defend and save them—is found in God. All else may fail, but the power of God will not fail. The result of all, therefore, should be to lead us to put our trust in God alone.