I can help share some information about the Duplicate functionality and work on resolving it.
If you're in the new estimate layout, the option is unavailable. You can revert to the previous layout if you still have the functionality or manually create a new estimate based on the existing one.
However, if you're in the old layout, yet the Duplicate option isn't visible in the drop-down menu, I can offer some troubleshooting steps to fix the issue.
Access your QuickBooks Online account using a private or incognito window. This mode doesn't retain your browsing history, occasionally leading to software issues. You can use these shortcut keys as a reference:
Ctrl + Shift + N for Google Chrome
Ctrl + Shift + P for Firefox
Control + Option +P if you're using Safari
If you're able to do so, return to the regular browser and clear the cache. This removes any historical data resulting in any error messages. Switching to a different supported browser can also fix the issue.
You can always reply in the comments below if you need further assistance managing estimates. We'll be right here, willing to help at any time. Have a good one!