Minn. Stat. § 475.69: DEFACED BONDS; DUPLICATES.

When any obligation of a municipality becomes unfit for circulation, it may be surrendered and canceled. Upon the authorization of the governing body, a duplicate of the obligation except as to signatures and a duplicate of any unpaid coupons, may be issued to the owners. These duplicates shall be marked "DUPLICATE" and the date of issue shown thereon. Such marking shall be signed by the treasurer then in office.