Prevent duplicate entries in a join table in a many-to-many relationship in JPA

I'm using EclipseLink 2.5.1 (and Hibernate 4.3.5 final). Given the following tables in MySQL.

• product
• prod_colour (join table)
• colour

There is a many-to-many relationship between products and their colours.

A product can have many colours and a colour can in turn be associated with many products. This relationship is expressed in the database by these tables.

The prod_colour table has two reference columns prod_id and colour_id from its related parent tables product and colour respectively.

As obvious, the entity class Product has a list of colours - java.util.List<Colour> which is named colourList.

The entity class Colour has a list of products - java.util.List<Product> which is named productList.

The relationship in the Colour entity :

public class Colour implements Serializable {

    @JoinTable(name = "prod_colour", joinColumns = {
        @JoinColumn(name = "colour_id", referencedColumnName = "prod_id")}, inverseJoinColumns = {
        @JoinColumn(name = "prod_id", referencedColumnName = "colour_id")})
    @ManyToMany(mappedBy = "colourList", fetch = FetchType.LAZY)
    private List<Product> productList; //Getter and setter.

    //---Utility methods---

    //Add rows to the prod_colour table.
    public void addToProduct(Product product) {
        this.getProductList().add(product);
        product.getColourList().add(this);
    }

    //Delete rows from the prod_colour table.
    public void removeFromProduct(Product product) {
        this.getProductList().remove(product);
        product.getColourList().remove(this);
    }
}

The relationship in the Product entity :

public class Product implements Serializable {

    @JoinTable(name = "prod_colour", joinColumns = {
        @JoinColumn(name = "prod_id", referencedColumnName = "prod_id")}, inverseJoinColumns = {
        @JoinColumn(name = "colour_id", referencedColumnName = "colour_id")})
    @ManyToMany(fetch = FetchType.LAZY)
    private List<Colour> colourList; //Getter and setter.
}

From the associated EJB, the insert operation is performed as follows.

@Override
@SuppressWarnings("unchecked")
public boolean insert(List<Colour> colours, Product product)
{
    int i=0;
    Long prodId=product.getProdId();
    for(Colour colour:colours)
    {
        Product p = entityManager.getReference(Product.class, prodId);
        colour.addToProduct(p);

        if(++i%49==0)
        {
            entityManager.flush();
        }
        entityManager.merge(colour);
        entityManager.merge(p);
    }
    return true;
}

Everything works fine.

When duplicate rows are attempted (same Colour entities associated with the same Product entity), they are also inserted into the prod_colour table that I expect should not happen.

Do I need to perform some extra conditional checks to avoid duplicate insertion or EclipseLink/Hibernate have some mechanism to prevent duplicates in such situations?