Al-Hasan (may Allah have mercy on him) says concerning this verse: "And do not retain them to harm them, so as to transgress" [Surah al-Baqarah: 231]: The husband would divorce his wife, take her back before the iddah ended, and then divorce her again; when the iddah was about to be completed, he would have witnesses to the taking back and then divorce her, so that her term would be made longer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15156