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I have checked your form, and you still have the default entry as your warning message when duplicate entries are submitted. To change that error message, you need to go to Preferences > Advanced Settings > Edit Form Warnings, scroll down to the following error, and click on it to edit it:
Sorry! Only one entry is allowed. Multiple submissions are disabled for this form.
Please refer to the image below for visual guidance:
After changing that error message and saving the form, I can see that a custom text is correctly applied. Here is an image from a cloned version of your form with this error message changed:
When you change the message of the Form Limits tab instead, where you have changed the message, that would be shown to your users when that form is disabled - not when there are multiple submissions. Please try changing the message in Advanced Settings > Edit Form Warnings instead, as shown in the image above, and see if this works to your satisfaction.
Let us know if you need further assistance, and we will be happy to help.