Polyptoton

Polyptoton is the repetition of a word root in a different grammatical form within the same passage — 'the judged were judging' — so the echo is heard as deliberate rather than accidental.

She had not been warned, and now she was the warning.

Warned and warning share a root and differ in form, and the shift is the point: the sentence turns a thing that happened to her into a thing she has become. That transformation is what polyptoton does, and no other figure does it as compactly.

More examples:

• He judged nobody, and was judged by everybody.
• The rules had rules.
• Nothing about the settlement settled anything.

It is worth putting next to mirror words, which are the same repetition committed by accident. The difference is not the frequency but the intent, and the reader can tell — a deliberate polyptoton falls on the stressed position of the sentence and is doing visible work, while an accidental echo sits in an unremarkable spot and simply chimes. If you are unsure which one you have written, move the repetition to the end of the sentence. If it earns the position, keep it; if it does not, it was an echo.

It sits naturally inside antithesis, since a contrast between two forms of one word is about as tight as a contrast can get.