This expression is brief and can be interpreted in various ways, but the interpretation I find most fitting is this:
“Once there was; once there was not.”
“Once there was” means that only God, the One, existed. “Once there was not” means that there was no other one God besides Him. In a sense, this can be understood as an interpretation of «فرد المتفرد», «احد المتوحد»: that absolute oneness belongs exclusively to the peerless Essence of the One. It may also be understood as an expression of the hadith «كان اللَّه و لم يكن معه شيء».
Of course, when the phrase “There was no one but God” is added to “Once there was; once there was not,” the matter becomes somewhat more complicated. Only this interpretation brings it closer to the intended meaning:
God, the One and Only, existed in His oneness; there was no one besides His Essence, and then He began creation. Even so, His Essence should not be counted as “one person” and His creatures as “other people.” The meaning, in truth, is this: “God, the One and Only, existed in His oneness, began creation, and remains in His oneness still.”