iCloud tabs are pretty great. I can have a website up on my laptop or iPad, and load it on my iPhone with ease. Recently I had to get my iPhone swapped out at the Apple store due to a hardware issue. When I loaded up the new phone and started browsing in Safari, I saw that my old phone was recognized as its own set of iCloud tabs.
Needing organization in my life, I went searching for a fix to clean things up. After a bit of hunting, I found this simple fix: patience. Basically you just need to wait it out, and Apple will automatically remove the old device in about a week.