What is the prescribed punishment that Umar ibn al-Khattab, may Allah be pleased with him, decreed for a woman who married while already married, and for the man who married her while unaware of her marital status?

Perhaps the questioner is referring to what Al-Qurtubi cited in his Tafsir (definition) from Masruq that Umar ibn Al-Khattab separated a man and a woman he married during her 'iddah (definition) (waiting period), and he placed her mahr (dowry) in the treasury. When this reached Ali, he said: "May Allah have mercy on the Commander of the Faithful. What is the matter with the mahr and the treasury? Indeed, they were ignorant, so the Imam should guide them back to the Sunnah (definition)." It was asked: "What then do you say concerning them?" He replied: "She is entitled to the mahr for what he made lawful of her private parts, and they should be separated. There is no flogging for them. She completes her 'iddah from the first husband, then she observes a full 'iddah of three menstrual cycles from the second, and then he may propose to her if he wishes." When this reached Umar, he addressed the people, saying: "O people, return ignorances to the Sunnah."