If this is a shared document, that may just mean what it says. If you're the only person editing it, something may have gone wrong on Google's end, and you can override the lock from that dialog.

You shouldn't override the lock if there's a possibility other people are editing citations in the document, or if you've opened the document in another tab and have been using Zotero there. If you're sure neither of those is the case, it's safe to do.

Well, in a large document, a Zotero operation can take a while (e.g., multiple minutes), but generally, yes. And if no one else is actually in the document, you're obviously fine.

I ran into a new problem. I went ahead and overrided the lock. When I tried adding citations, I saw my references at the end duplicate. Is there another underlying problem here?