From: Rabbi Moshe Sofer
To: Rabbi Mordechai Leib
A Kohen who unknowingly married a gerushah finds himself bound to an absent wife who has fled, preventing him from fulfilling piryah v'rivyah. The Chasam Sofer rules that zikui get is inapplicable since divorce confers no tangible benefit on a woman not suspected of promiscuity, and therefore the husband must obtain a heter meah rabbonim to marry a second wife, depositing a get for the absent wife.
Source: Shu"t Chasam Sofer EH I §3