We haven't received any reports regarding any issues about duplicate customers displaying in the search field in QuickBooks Online (QBO).
When QuickBooks has unexpected behavior, let's perform some basic troubleshooting steps to fix the problem since sometimes cache stored up in the system can cause unexpected behavior in the product. You can start by opening a private (incognito) window. Using a safe browser and accessing the program from there can usually fix the issue.
Here's how to open it:
Once done, try open your QBO account again and search customers. Check to see if it works. If yes, then your main browser causes this kind of issue. To ensure this doesn't happen again, it's a good idea to clear the cache of the browser you're using.