How to replace existing workflow (activity and transition) using import method

Hi

i would like to replace existing workflow like activity and transistion in administration - > customization - > workflows

i use this kind of code to import it
<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="act_confirmed" model="workflow.activity">
            <field name="wkf_id" ref="purchase_order"/>
            <field name="name">confirmed</field>
            <field name="kind">function</field>
            <field name="action">wkf_confirm_order()</field>
        </record>
        <record id="act_waitingapproval" model="workflow.activity">
            <field name="wkf_id" ref="purchase_order"/>
            <field name="name">waitingapproval</field>
            <field name="kind">function</field>
            <field name="action">wkf_waitingapproval()</field>
        </record>
        <record id="act_waitingmaterialsupervisor" model="workflow.activity">
            <field name="wkf_id" ref="purchase_order"/>
            <field name="name">waitingmaterialsupervisor</field>
            <field name="kind">function</field>
            <field name="action">write({'state':'confirmed'})</field>
        </record>
        <record id="trans_waitingmaterialsupervisor_confirmed" model="workflow.transition">
            <field name="act_from" ref="act_waitingmaterialsupervisor"/>
            <field name="act_to" ref="act_confirmed"/>
            <field name="signal">purchase_approve</field>
        </record>
        <record id="trans_waitingapproval_confirmed" model="workflow.transition">
            <field name="act_from" ref="act_waitingapproval"/>
            <field name="act_to" ref="act_confirmed"/>
            <field name="signal">purchase_approve_direct_supervisor</field>
        </record>
        <record id="trans_waitingapproval_waitingmaterialsupervisor_confirmed" model="workflow.transition">
            <field name="act_from" ref="act_waitingapproval"/>
            <field name="act_to" ref="act_waitingmaterialsupervisor"/>
            <field name="signal">purchase_approve_material_supervisor</field>
        </record>
    </data>
</openerp>

but there is one thing that is weird here, id="act_confirmed" is an existing "activity".
If i don't place it in here, when i importing, it will command an error that said couldn't find act_confirmed, therefore i added here.

but if i added here, when i import the module to OpenERP, the activity will be duplicated, and the workflow will also be messed up. I don't want it to be duplicated.

Anyone have clue about this? how to replace it? or what solution best for this. Thanks