From: Rabbi Moshe Sofer
To: Rabbi Ber
When the majority of men in an area are disqualified for kehunah, a woman suspected of illicit relations cannot be permitted to marry a Kohen. However, when the population is evenly split, a valid sfek sfeika applies permitting the marriage, provided she lived in a distinctly Jewish neighborhood where non-Jews did not frequent, the courtyard was locked at night, and an additional distinguished rabbi concurs.
Source: Shu"t Chasam Sofer EH II §122