Dual citizenship
Section 28 · 1999 Constitution (2004 edition)
What Constitution says
(1) Subject to the other provisions of this section, a person shall forfeit forthwith his Nigerian citizenship if, not being a citizen of Nigeria by birth, he acquires or retains the citizenship or nationality of a country, other than Nigeria, of which he is not a citizen by birth. (2) Any registration of a person as a citizen of Nigeria or the grant of a certificate of naturalisation to a person who is a citizen of a country other than Nigeria at the time of such registration or grant shall, if he is not a citizen by birth of that other country, be conditional upon effective renunciation of the citizenship or nationality of that other country within a period of not more than five months from the date of such registration or grant.
This is the 1999 Constitution as published in 2004. Later amendments aren't included yet, so a section may have changed since.
Original source
Official Gazette · CFRN 1999 official PDF · printed page 32. Original document
Release: draft-c8d50d87fae9. Source scope, review and corrections