I automatically use ConcurrencyMode.Multiple for my WCF services because I usually expect the service to be able to handle multiple requests at the same time. For me, this seems to be the most intuitive behavior for the service (i.e. it would be strange if I called the web service and had to wait 10 outstanding requests from other users before they visited me).
Using ConcurrencyMode.Single forces the service to process one request at a time (in the case of multiple requests). This may slow down requests.
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