Hi Ghoztrider,
This one's a bit tricky. I've written down a couple of steps for you to follow:
• Add a Validity Boolean field with a default value of TRUE
• Add a retrieve data connection to the List
• When the field is changed, execute these actions:
1. Use the entered Item as a filter value in the List data connection, and query the data.
2. set Validity to the value of this formula: not(SharePointListItem)
- where SharePointListItem is the repeating group from that connection
• Add a calulated value box with this text with an error message - hide it when Validity is true.
• Disable the Submit button when Validity is not true.
I'm also attaching a sample form for reference. Hope this helps.