Please try the following:

a. Make a backup copy of the K2HostServer.Exe.Config file in program files (x86)K2 blackpearlHost ServerBin folder to another folder (maybe on desktop or c: drive)
b. Open the K2Hostserver.exe.config file in a notepad
c. Locate:-
<sourceCode.smartActions.directoryCatalog>
<catalogs>
<add path="SmartActions" />
<add path="SmartActionsEWS" />
<add path="SmartActionsSMTP" />
</catalogs>
</sourceCode.smartActions.directoryCatalog>

Is there both EWS and SMTP Entry?
d. If so, try commenting out <add path="SmartActionsSMTP" /> (i.e. <!-- -->)
e. Save the file and restart the K2 blackpearl server service

f.  Test if still receive double notifications.

Thanks Tin, this is exactly what the problem is, thanks for your prompt response.