I'm not sure if this a hardware or software issue.  I recently installed a FocusRite Scarlett 18i20 audio interface and have 8 analogue inputs plugged in.  A Korg Kronos88 is plugged into inputs 1 and 2.  A Roland sound module is plugged into inputs 3 & 4.  (Equipment plugged into inputs 5-8 is turned off and not part of the problem.) 
 
I am using SONAR Platinum on a 64-bit computer built by Studio Cat specifically as a DAW.
 
On playback, when all inputs are selected in the FocusRite Control interface, the Korg and Roland play through MIDI tracks fine but the Korg also plays a second note for each MIDI note when the SOFTWARE DAW 1 & 2 OUTPUTS are selected.  I have all audio tracks muted so it's not playing both tracks simultaneously.  I haven't been able to determine if a setting is wrong in Sonar or in the FocusRite interface.  Also, I apparently do not understand something about the interface because I can get things to work by de-selecting outputs but then I can't use those outputs for audio tracks simultaneously with MIDI.  I need both for most recordings and audio export.

A few questions:
 
When you say the Korg plays a second note, is it the same pitch, and is it layered on top of the first causing phasy sound, or is there a distinct delay/latency?
 
Where is "SOFTWARE DAW 1 & 2 OUTPUTS are selected" happening? in the FocusRite control panel/mixer app, on the front panel or as an audio track input to SONAR/CbB?
 
Are you input-monitoring the synths via audio tracks in SONAR (recommended) or direct-monitoring through the interface? If the second note has a short but distinct delay, you might be doing both, and you need to disable one or the other monitoring path.
 
How is MIDI routed to the Korg - via direct USB connection or via MIDI OUT on the FocusRite?
 
If the sound is phasy, that would more likely be a problem with dupe MIDI notes reaching the Korg.
 
I'm not familiar with FocusRite's interfaces, so I can't give specific guidance there.
 

Try checking the Korg is set to “local off” - it might be sending a MIDI note to the DAW which then gets echoed back to the synth and triggers a repeat note.

brundlefly
A few questions:
 
When you say the Korg plays a second note, is it the same pitch, and is it layered on top of the first causing phasy sound, or is there a distinct delay/latency?
 
Where is "SOFTWARE DAW 1 & 2 OUTPUTS are selected" happening? in the FocusRite control panel/mixer app, on the front panel or as an audio track input to SONAR/CbB?
 
Are you input-monitoring the synths via audio tracks in SONAR (recommended) or direct-monitoring through the interface? If the second note has a short but distinct delay, you might be doing both, and you need to disable one or the other monitoring path.
 
How is MIDI routed to the Korg - via direct USB connection or via MIDI OUT on the FocusRite?
 
If the sound is phasy, that would more likely be a problem with dupe MIDI notes reaching the Korg.
 
I'm not familiar with FocusRite's interfaces, so I can't give specific guidance there.
 

It could be either or both a audio/MIDI issue.
 
Audio:
You've got a choice, either monitor what your Kronos is playing, or monitor the sound you're recording. You don't want both.
 
If you want to directly monitor what is coming from the Kronos, turn off the track monitoring on your track within Sonar.
 
If you want to just monitor what you're recording, make sure track monitoring is enabled for the recording track within sonar, but mute tracks 3&4 within Scarlett MixControl (make sure 3 & 4 are still routed to the DAW inputs though).
 
MIDI:
A similar rule applies here. Either you want to hear the result of the MIDI you've played, or the MIDI Sonar is echoing back to you. You don't want both.
 
If you're recording MIDI on to a track, then either turn off MIDI echo on that track, or turn off local control on your Kronos. Local control stops the Kronos keyboard from playing its own internal sounds, instead relying on the MIDI echo coming back from Sonar.
 
 
 
 

FWIW, my reading of the issue is that it involves playback of existing MIDI tracks, not real-time performance, so Local Control wouldn't be relevant.

I appreciate all the feedback. The problem is not resolved yet.
 
To answer brundlefly, the second note is the same pitch as the first and there is a slight delay.  "SOFTWARE DAW 1 & 2 OUTPUTS" are on the FocusRite Control interface (in the ap, not on the hardware).  I do not have monitors connected to the interface.  I have the outputs connected to 5.1 speakers on 6 individual output channels.  Outputs 1 & 2 drive front right and left speakers, Outputs 3 & 4 drive rear right and left speakers and Outputs 5 & 6 drive a center channel & sub.  All of this is working just fine.  MIDI is routed to the Korg (and the Roland) though a Roland UMG3 MIDI interface.   No MIDI cables are connected to the FocusRite audio interface.
 
To answer tlw, I did turn local MIDI control off on the Korg but that has no effect on the problem.
 
To answer msmcleod, all audio tracks are muted.  In fact, I solo-ed the Korg MIDI track and witnessed the problem with no other tracks playing.
 
I changed all the (muted) audio outputs to FocusRite Stereo 5 just to make sure I was not using the same Software DAW outputs in the FocusRite interface ap as the offending one (Stereo 1) that causes the repeating notes.  This made no difference.  I also have the echo turned off on the Korg MIDI track so that doesn't explain it either.
 
Hopefully these additional details shed some light on the problem I'm having and why it is making no sense to me.  I'm not a recording engineer but I have worked with SONAR/Cakewalk for a very long time and used a simpler FocusRite interface (a 6i6) before without experiencing a problem like this.
 
Lastly, I found out that with the Korg's local MIDI control turned on OR off, notes that I PLAY on that board are also repeated.  Does anyone have additional ideas on what's going on and how to fix it?
 
Thanks,
UCG Musician

Check the MIDI routing on your interface to see if what your sending out from the Kronos is not being re-routed back into the Kronos. If so then, go to the global settings on the Kronos and then select "page select". Find "local control on" and un-check that box.
I have the same issue sometimes with my Korg M3.
 
 

Open your midi track in Staff View. 
 
Any doubled notes will be immediately apparent. This will determine whether it's a monitoring problem or a recording problem,

There are no double notes in staff view.  I don't know what setting is enabling input monitoring.  MIDI Echo is off.

I think I may have resolved the problem myself.  It appears to be a setting in the Windows Volume Control.  When I mute the audio interface as an application but keep it unmuted as a device the double note issue stops.  If anyone can explain why this would be the case, I'd appreciate the insight.  I know it's no longer a SONAR issue in that event.