Re: Overlapping Copy-Paste creates zombie clips (bug since X3)
I'm guessing you have Drag and Drop mode set to 'Slide Over to Make Room' in Preferences > Customization > Editing? That setting has had issues going back before X3. I can't replicate anything like what you describe with 'Blend' mode selected, which is what you would want for crossfading overlapped clips.
SONAR Platinum x64, 2x MOTU 2408/PCIe-424 (24-bit, 48kHz)
Win10, I7-6700K @ 4.0GHz, 24GB DDR4, 2TB HDD, 32GB SSD Cache, GeForce GTX 750Ti, 2x 24" 16:10 IPS Monitors