I am almost cringing at asking this but.

Lets say I have recorded a drum session using SD2 and i may have used my control keyboard to hit a few notes (Korg Triton LE) but often just used the SD midi patterns.

Then later I want to record some Dimension Pro or other softsynth midi tracks.  I get the SD3 being triggered by the Korg...so with every note I get a cymbal or a tom etc.

I usually solve this by some random changing of input  or midi channels but it is all guess work.

Simple question how do I stop it by the logical method and so I can always understand what's happening nd simply switch off the correct switch!

That was badly explained but there you go.

Check if the echo button of the Session Drummer tracked is on ? (next to record button, turned to blue color ).

Every VSTi needs a MIDI channel of its own. Otherwise everything using, say, MIDI channel 1 gets triggered when any one of VSTis using that channel gets triggered.

And you most likely (don't know the Korg) must switch the channel in Korg as well. If the VSTis in SONAR are using channels 1,2,3... chronologically, and Korg is sending messages on channel 1, then
channel 1 is what you hear.

pretty much all h/w synths default to midi ch 1 for their keyboard parts...  the simplest solution is to control the midi reception by removing input echo as tomixornot suggests, or you can setup a multi timbral performance in the triton and control the midi transmission of the synth.. (its a good idea to work in multi-timbral mode anyway since you can setup splits & layers etc)

Thanks a lot...appreciated will certainly check the input echo