Finland allows dual citizenship. U.S. citizens residing in Finland who are interested in applying for Finnish citizenship should be aware of the following guidance on U.S. nationality policy:
Information on dual nationality
Entering and Departing the U.S.
According to Section 215 of the U.S. Immigration and Nationality Act (8 U.S.C. 1185), it is illegal for an American citizen to enter or leave the U.S. on anything other than an American passport. This applies to dual citizens as well, meaning that persons holding e.g. both Finnish and American citizenships and passports must enter and leave the U.S. on a U.S. passport. They may not enter/leave the U.S. on a Finnish passport. This applies to children as well as adults.