Must one repeat the Dhuhr prayer if one began it thinking the time had commenced, then heard the Adhan during the second Rak'ah?

Knowledge or strong presumption that the prayer time has begun is a condition for its validity. The questioner must investigate by asking trustworthy individuals about the prayer time or by using reliable calendars. If it becomes clear that she prayed after the time began, her prayer is valid. However, if it becomes clear that she prayed before the time began, she must repeat it.