@wroos, let me try to explain this. When you select Online-Only (single submission) you are allowed to make a single submission with the URL. You will then see this message as shown in the image below after submitting the submission to the server:
You should be able to make a new submission with the same URL in the same browser (if you paste the URL in a new tab to make a new submission) when this option is selected.
However, when you select Online-only (once per respondent), you are only allowed to make a single submission with the URL. You will then see this message as shown in the image below after submitting the submission to the server:
With this option selected, you can only make a single submission with the URL. Pasting the URL in a new tab to make a new submission in the same browser should not work with this option unless you change your browser or clear your browser’s cache. You should get a similar message as shown in the image below when you paste the URL to make a new submission (in the same browser) again:
These details should also provide you with additional hints:
