Is the Islam of one who uttered the Two Testimonies while simultaneously speaking to herself valid, or must she repeat them?

If your friend pronounced the Two Testimonies (Shahadatayn) while understanding their meaning, then she has entered Islam. The fleeting thoughts she experienced during the pronouncement do not harm her conversion, for the absence of complete mental focus does not invalidate the Shahadatayn nor necessitate their repetition. It is not a condition to fully conceptualize their meaning at the moment of pronouncement; rather, the condition is that one does not intend anything other than their true meaning. The most complete way is for the verbal pronouncement to be accompanied by the heart's remembrance. Therefore, one should not pay attention to doubt, as it stems from the whispers of Satan. If she desires further reassurance, there is no harm in repeating the pronouncement of the Shahadatayn, but this is not obligatory.