You can very well have two different endpoints for the same service- one with pollingduplex and other one with wsdualhttpbinding.
in config:
<services>
...
<service name="dfds">
<endpoint
binding = polling
address="something else"
contact="samecontract"
></endpoint>
<endpoint
address="something"
binding = wsdual
contact="samecontract"
></endpoint>
</service>
</services>
<br/>
HTH,
-Phani
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