When I right click on an audio track's effects bin and then click Insert Audio FX, many of the VSTs which are listed appear twice. 
Why is that?
It's not a 32 bit, 64 bit thing. The plugins don't list as 32 bit.
 
Let's take UAD Ocean Way for example. It appears twice in the list. So I did a search of UAD Ocean Way on my computer, and I have 2 instances of the .dll file, both 64 bit...UAD Ocean Way Studios.dll, and UAD Ocean Way Studios(m).dll. The m apparently means mono.
 
But when I instantiate the Ocean Way plugins in the effects bin, they both show as stereo.
 
Thanks.
 
 

I think (not sure) the "M" versions of UA and other plug-ins are mostly for Pro Tools people. You can just exclude them in the plug-in manager.

Thanks, but I don't think that totally clarifies the issue, as evidenced by the fact that both plugins instantiate as stereo. They both have the 2 vertical lines hanging down from the bottom.
 
But your reply raises an interesting question: Is there any use for mono plugins in Sonar?

Some are VST 3 listed along with VST2s!

Hangdog Cat
But your reply raises an interesting question: Is there any use for mono plugins in Sonar?

 
Yes, it is best to keep the mono (m) versions and use them on mono tracks. SONAR is a bit peculiar in that unlike most other DAW apps (Cubase, Studio One, etc..) using a stereo version on a mono track does not reduce the DSP usage. It is treated as a stereo plugin as far as SONAR is concerned and the UAD devices will use the full DSP load.
You will need to remember to use the interleave button to change from the default stereo to mono (watch the pins in the FX bin to show the status). But the DSP usage will NOT change. There can be a dramatic difference in DSP usage between stereo/mono versions. For example a stereo FATSO uses 40.5% DSP per instance. A mono instance will use 25.6% per instance. 
 
I keep both stereo/mono versions installed and use them appropriately on stereo/mono tracks to minimize DSP usage. In regards to the UAD plug-ins showing twice without being able to distinguish them, you will need to edit the cakewalk.ini file to change how SONAR reads and show the plugin names. See the following thread as to how this is done and why. Set the variable to 1 for the old behavior (prior to SONAR X2a).
 
http://forum.cakewalk.com/Missing-plugin-suffixes-SOLVED-m3204949.aspx#3505682
 
Cheers,
 
Billy Buck
 
 

I did a second scan for the Ocean Way .dll and found a third instance. I must have missed it the first time. It turns out I have each UAD plugin in 2 places, 1 in the Steinberg folder, and 1 in the UAD folder. I think I'm stuck with this arrangement, because I don't know which plugins I used in my past projects. 
 
I just updated UAD yesterday, and apparently it updated the plugins in both folders. 

I found that sometimes you might want to combine different mono vocal parts using a Mono Channel interpolation (yes like my name lol). My UAD doesn't seem to have any Mono plug-ins listed which I haven't investigated. 
 
VST plug-ins can be certainly set to run as mono or even if it's VST 3 they seem to be handle this better, seeing as not every channel is more than one. Actually I am going to try and address this mono plug-in issue now.
 
 

The following link illustrates the aforementioned behavior. I have inserted two stereo instances of the FATSO (tranny enabled) into a SONAR project. One on a mono track (with mono interleave enabled) and the other on a stereo track. Both use the same amount of DSP per instance for a stereo instance (40.5% x 2 = 81% combined). The mono track should be using 25.6%. Only when you insert the proper (m) mono version will the reduced DSP take affect. This behavior has been the same since I first started using UAD plugins with SONAR back in 2003!!!! I watch my UAD DSP load like a hawk and intimately know how the UAD plugins act when using SONAR and how to minimize DSP usage. Not using the (m) mono versions on mono tracks is just wasting valuable DSP.
 
 
https://1drv.ms/i/s!ApH0AmpE3sG2bUwf8dx5ij1K8d4
 
 

Billy Buck
 ... 
http://forum.cakewalk.com/Missing-plugin-suffixes-SOLVED-m3204949.aspx#3505682
...

MANY THANKS for this link - I have been going through every UAD plugin and editing the names to add a "MONO-" prefix so I can tell them apart easier - every time UAD software gets updated, a tad tedious. 
 
I also move the not-purchased-but-always-installed UAD plugins to an unscanned folder as well, not quite manually but have set up a spreadsheet and scripts to do it, still seems there ought to be a better way :)
 
Thanks again!

Can't see the image and the link is broke. OneDrive is notorious for this. I quit using it after finding I can't even add pics to a "good" folder and have them show.