Windows service in multiple server

I have created a windows service which is installed in '6' app server, The windows service will pick the data from db and send an email, after sending it will update the send status as '1' which will be initially '0'.

In some cases the service is picking up the same set of data by different app server and sending email. so the same mail is sending multiple time.

How can I fix this issue so that the server will not pick the same set of data?