Here's what you can do:
- Detect and prevent duplicates across repeat instances in one form: Avoiding duplicate selection in repeat group - #22 by LN
- Use
last-savedin a similar way to prevent duplicates across local submissions of the same form: Deactivate option if selected once - #3 by ahblake (be sure to read about limitations) - Use the entity preview or an attached CSV updated based on previously-submitted data to prevent duplicates across all submitted data (requires devices being frequently online)
If you describe your workflow in more detail, there may be other ideas for you.