Negative Mitzvah 356
It is forbidden to remarry one's divorced wife who married and divorced a second time
Deuteronomy 24:4 "Then her former (first) husband who sent her away may not take her again to be his wife"
A man who divorces his wife is forbidden to remarry her if she married another man and then, was divorced a second time or was widowed from her second husband.
(If she did not marry another man, he is permitted to marry her again.)