<?xml version="1.0" encoding="UTF-8" ?>

<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
                  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                  xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                                      http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

    <entity name="Sylius\Bundle\CoreBundle\Model\Variant" table="sylius_variant">
        <id name="id" column="id" type="integer">
            <generator strategy="AUTO" />
        </id>

        <field name="sku" column="sku" type="string" nullable="true" />
        <field name="price" column="price" type="integer" />
        <field name="onHand" column="on_hand" type="integer" />
        <field name="availableOnDemand" column="available_on_demand" type="boolean"/>

        <one-to-many field="images" target-entity="Sylius\Bundle\CoreBundle\Model\VariantImage" mapped-by="variant">
            <cascade>
                <cascade-all />
            </cascade>
        </one-to-many>
    </entity>

</doctrine-mapping>

<?xml version="1.0" encoding="UTF-8" ?>

<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
                  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                  xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                                      http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

    <entity name="Sylius\Bundle\CoreBundle\Model\Variant" table="sylius_variant">
        <field name="sku" column="sku" type="string" nullable="true" />
        <field name="price" column="price" type="integer" />
        <field name="onHand" column="on_hand" type="integer" />
        <field name="availableOnDemand" column="available_on_demand" type="boolean"/>

        <one-to-many field="images" target-entity="Sylius\Bundle\CoreBundle\Model\VariantImage" mapped-by="variant">
            <cascade>
                <cascade-all />
            </cascade>
        </one-to-many>
    </entity>

</doctrine-mapping>

Remove the duplicate id mapping from Variant model
Remove the duplicate id mapping from Variant model
