#JavaScript (ES6), 25 bytes

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    s=>/(.+?)\1*$/.exec(s)[1]

[Try it online!][TIO-k2nvzdcg]

[JavaScript (Node.js)]: https://nodejs.org
[TIO-k2nvzdcg]: https://tio.run/##y0osSyxOLsosKNHNy09J/Z9m@7/Y1k5fQ0/bXjPGUEtFXy@1IjVZo1gz2jD2f3J@XnF@TqpeTn66RpqGUo0aBCppanKhSVmp1UAQFjkNDbUaZAxU8h8A "JavaScript (Node.js) – Try It Online"