When would a duplicate foreign key be used?
I think there's no practical purpose in such duplicate foreign keys. Some RDBMSs even do not allow these FKs. For example, you'll get an error message "ORA-02275: such a referential constraint already exists in the table" if you run the following code in Oracle:
CREATE TABLE parentTbl ( id INT UNIQUE );
CREATE TABLE childTbl ( child_id INT UNIQUE, parent_id INT);
ALTER TABLE childTbl ADD CONSTRAINT FK1 FOREIGN KEY (parent_id) REFERENCES parentTbl(id);
ALTER TABLE childTbl ADD CONSTRAINT FK2 FOREIGN KEY (parent_id) REFERENCES parentTbl(id);
The tables parentTbl, childTbl and constraint FK1 will be created, while the creation of FK2 will fail.