How to reproduce
- Create a dummy Elements Connect field
- In the Edit View, make it Read-Only - Single Value
- Go in the Search View, and configure a specific template
- Click on Advanced options
- "No duplicates" option is available
- "No duplicates" option is not available
- The "No duplicates" option is not available for Read-Only fields in Single Value because it's not relevant. But, the option should be still proposed in the Search View as the context is different, and the option could be useful.