Before posting this, I looked through other threads about this issue (duplicate request errors) and read the relevant documentation:
That page gives the following advice:
If you are seeing this warning repeatedly, check the Performance History to see why the client Sync requests is taking so long to complete. This warning may signify that the add, update, or delete is taking a long time to process. It may also signify that the add, update, or delete is triggering a workflow rule that is taking a long time to process.
Checking the performance history (see below) wasn’t particularly useful. The “fix” that worked for me was trying to do something else on the app to add more data to the queue. All of this occurred on the Safari browser (not mobile phone).
This strikes me as an issue on the AppSheet side. Am I right about that? Also, adding more data to the queue was easy enough in my case. For other users, however, this fix might not work.