Open the respective dashboard. Click on the ‘Configure Board’ option from the left pane.
Under ‘Alert Settings’ section, expand the ‘Advanced Settings’
In the 'Content Settings' section, select the "Remove duplicate updates from the alert mail" checkbox and Click on "Save"
By using this feature, if an update is a part of multiple widgets on the dashboard, then it will be sent only once in each alert mail (under the first widget it matched against).