Matron Landslide
Well done, just tried that, and it does indeed happen if you add the vst2version and vst3 version of same plug.

In theory, I could see that happening. Unfortunately, for the plug-in in question, I have only the 64-bit, VST2 version (there is no other version, other than AAX), and it's listed twice. I only have the plug-in installed in one location on the PC, and I uninstalled it thoroughly before upgrading it just before I updated SONAR. When I re-installed it, it had the same ID as the previous one, and I didn't even need to use the Plug-in Manager to find it. SPlat's never listed this plug-in in any list twice, so it still makes no sense as to why it's doing it now.
 
So far, I've only seen SPlat do this with one plug-in, but I haven't used it on too many projects since the update, so I don't know how it will continue to behave.
 
For those of us who compose, record, mix and/or master in different genres, the "last-used" plug-in (more often than not) is not the one you need for the next project. (A "last-used" list full of amp sims doesn't help me at all when I need to mix a synth project.) As others have suggested, letting us put our "favorite" plug-ins in that list might help more.
 
Steve

Matron Landslide
Well done, just tried that, and it does indeed happen if you add the vst2version and vst3 version of same plug.

Also, if you have the VST2 and VST3 versions installed, isn't SONAR supposed to default to the VST3 version (per "VST3 Migration" in the Preferences)?
 
In theory, you should only see both versions in the list of synths/effects the Manager finds during a scan; you shouldn't see VST2 versions in your list of plug-ins to choose once you're back in Splat. So I'm not sure how you'd be able to choose both, unless you turned off the default, or you manually re-named or added the VST2 version, to make sure you could choose which one you wanted (which I've done for a couple of synths/effects).
 
Steve

Simple, I have 'Hide related VST2 plugins' unchecked in preferences, always have had it that way since there was a setting to do so. I rarely use SONAR now days, it's not my maim DAW of choice, but I keep an eye on it.

Matron Landslide
Simple, I have 'Hide related VST2 plugins' unchecked in preferences, always have had it that way since there was a setting to do so. I rarely use SONAR now days, it's not my maim DAW of choice, but I keep an eye on it.

And that's fine. But with it set at the default, Sonar should hide the VST2 version. So in your case, you'll see both. But there's still no explanation why I see two of the same, single VST2 plug-in twice. (Of course, I'm not expecting you to figure out why this is happening; I'm only mentioning this for clarification.)
 
Basically, I know how Sonar is supposed to work--I just don't know why it's showing me the same plug-in twice.
 
Just for grins, I clicked on each of the two entries in the "recently used" list. Sonar opened two instances of that plug-in (which I've never had in a project). So it really is listing it twice.
 
Fortunately, it didn't add those two instances to the list, too. 
 
Steve

I think that the duplicate entries may have been to do with when the list hadn't ever been fully populated. I had duplicates showing in the menu to begin with and thought WTF, but since I have now used more than different five plug-ins it doesn't seem to do that any more. Either way, it's an unnecessary waste of space IMO and if there was a way to switch it off, I would.
For those who want a go to list, it has always been easy enough to have the plug-ins displayed however you want them to be, so this seems to be very much a case of fixing something that wasn't broken whist still ignoring things that are.
 
 

To return to the OP's question, I've found you can disable it with seemingly no bad side-effects, although it does require registry editing:
 
Ensure the user/group you are running SONAR under does NOT have "Create Subkey" privileges to the following two registry keys HKEY_CURRENT_USER\Software\Cakewalk Music Software\SONAR\Platinum\Recently Used Plug-ins and HKEY_CURRENT_USER\Software\Cakewalk Music Software\SONAR\Platinum\Recently Used Plug-ins
 
I had to fiddle with mine a bit and use explicit Deny permissions as the user I run under is in the Administrator group.

It also doesn't display the plugin's name correctly as I have changed them in my custom menu layouts.
I use Steinberg's way of marking vst3 ( /// ) for certain plugins as I sometimes need to use vst2. It only shows the scanned name so I have no way of knowing which is being used.

All in all, this appears to be a well-intentioned solution in search of a problem. Hopefully, Cakewalk will give us the option to not have to edit the Registry just to remove what we didn't want in the first place (not that I mind doing that, thank you, @KPerry). And if we do want it, hopefully they'll find a way to have the list use the VST names we use and that we're familiar with, instead of whatever it's using now.
 
Steve

I for one love this lil feature.  I agree there should be a disable button options are great.

Generally, I'll "test" several plugs before deciding which one works best for the track. In this situation, the list is always evolving, therefore a total waste of real estate. "Last Used" doesn't mean I want to use it again.