This annotation:

    /**
     * @hibernate.many-to-one class="pl.addcom.df.Employee" name="deputy"
     * column="deputy_id" not-null="true" cascade="none"
     * unique="true"
     *
     * @return Returns the deputy.
     */

produces that entry in the hibernate mapping file:

       <many-to-one
            name="deputy"
            class="pl.addcom.df.Employee"
            cascade="none"
            outer-join="auto"
            update="true"
            insert="true"
            unique="true"
            column="deputy_id"
            not-null="true"
            unique="true"
        />

The problem is duplicated "unique" attribute, and this in turn causes schemaexport task to end with an error.