Although this might be used often by some... I have had nothing but issues.
I have found I have corrupted profiles after I have done changes to profiles when you have multiple windows of S3D open and so maybe the changes sometimes screws up profiles... well this is just my latest thoughts... other than it's just the fact I have either so many profiles or some other issue happening.
I have 8 3D printers running and they all bar 2 are different... so I have 7 profiles and within that many versions of that for filament types etc. So I have 60+ profiles and since I have been diligent in making sure I only have one instance of S3D open... I have managed to stop screwing up profiles... well I think this is it anyway.
I recently stumbled onto this program. https://www.dcmembers.com/skrommel/down ... einstance/
It seems to work fine at this point (day one of use). In the script file SingleInstance.ini change the line closeprograms= to add the S3D program file. For me it is closeprograms=\Program Files\Simplify3D-4.1.2\Simplify3D.exe
If you click on the icon to start another instance it opens then closes again. Same as if you double click on a STL file (or any associated file) it will open then quickly close leaving you with just the one instance open.
If anyone has any other program doing the same id like to know what else is out there.