Error Code 1022 in MySQL - mysql

Question

I'm pretty new to database processing. I'm trying to create table
ASSIGNMENT (ProjectID, EmployeeNumber, HoursWorked)
ProjectID and EmployeeNumber are Composite Primary Keys of table ASSIGNMENT and Foreign Keys of tables PROJECT and EMPLOYEE (see below)
Here is my data:
CREATE TABLE IF NOT EXISTS ASSIGNMENT(
ProjectID Int(4) NOT NULL AUTO_INCREMENT,
EmployeeNumber Int(4) NOT NULL,
HoursWorked Int(4) NOT NULL,
PRIMARY KEY (ProjectID),
UNIQUE (EmployeeNumber),
KEY ProjectFK (ProjectID),
KEY EmployeeFK (EmployeeNumber),
CONSTRAINT ProjectFK FOREIGN KEY (ProjectID) REFERENCES PROJECT(ProjectID) ON DELETE CASCADE,
CONSTRAINT EmployeeFK FOREIGN KEY (EmployeeNumber) REFERENCES EMPLOYEE(EmployeeNumber)
);
I've seen a lot of examples like this one but what makes this instance unique is that MySQL doesn't allow two Primary Keys in one table so I made EmployeeNumber a UNIQUE key.
Here is the data for the entire schema:
CREATE SCHEMA IF NOT EXISTS WPC;
CREATE TABLE IF NOT EXISTS DEPARTMENT(
Department Char(30) NOT NULL DEFAULT 'Human Resources',
BudgetCode Int(20) NOT NULL,
OfficeNumber Int(10) NOT NULL,
Phone Char(12) NULL,
PRIMARY KEY (Department));
CREATE TABLE IF NOT EXISTS EMPLOYEE(
EmployeeNumber Int(4) NOT NULL AUTO_INCREMENT,
FirstName Char(25) NOT NULL,
LastName Char(25) NOT NULL,
Department Char(30) NOT NULL,
Phone Char(17) NULL,
Email VarChar(100) NOT NULL,
PRIMARY KEY (EmployeeNumber),
UNIQUE KEY Email (Email),
KEY DepartmentFK (Department),
CONSTRAINT DepartmentFK FOREIGN KEY (Department) REFERENCES DEPARTMENT(Department) ON DELETE RESTRICT ON UPDATE CASCADE
);
CREATE TABLE IF NOT EXISTS PROJECT(
ProjectID Int(4) AUTO_INCREMENT,
ProjectName Char(20) NOT NULL,
Department Char(30) NOT NULL,
MaxHours Int(14) NOT NULL DEFAULT 100,
StartDate Char(10) NOT NULL,
EndDate Char(10) NULL,
PRIMARY KEY (ProjectID),
KEY ProjectFK (Department),
CONSTRAINT ProjectFK FOREIGN KEY (Department) REFERENCES DEPARTMENT(Department) ON DELETE RESTRICT ON UPDATE CASCADE
)
ENGINE=InnoDB AUTO_INCREMENT=1000;
SET ##AUTO_INCREMENT_INCREMENT=100;
CREATE TABLE IF NOT EXISTS ASSIGNMENT(
ProjectID Int(4) NOT NULL AUTO_INCREMENT,
EmployeeNumber Int(4) NOT NULL,
HoursWorked Int(4) NOT NULL,
PRIMARY KEY (ProjectID),
UNIQUE (EmployeeNumber),
KEY ProjectFK (ProjectID),
KEY EmployeeFK (EmployeeNumber),
CONSTRAINT ProjectFK FOREIGN KEY (ProjectID) REFERENCES PROJECT(ProjectID) ON DELETE CASCADE,
CONSTRAINT EmployeeFK FOREIGN KEY (EmployeeNumber) REFERENCES EMPLOYEE(EmployeeNumber)
);
ProjectID and EmployeeNumber are composite primary keys in the table ASSIGNMENT. They are also Foreign Keys that reference tables PROJECT and EMPLOYEE.
Everything with the database is fine except for the ASSIGNMENT table. When I run the script for ASSIGNMENT I get this response:
Error Code: 1022. Can't write; duplicate key in table 'assignment'.

Answer 1

The names used for Foreign Key constraints must be unique in the database. You're attempting to use the same name for FKs on different tables.
You're using the name ProjectFK on the PROJECT table, and attempting to use the same name again on the ASSIGNMENT table.
Change the FK constraint name on one of the tables.