I set up an Airtable evaluation account literally 10 minutes ago and have already run into this critical yet seemingly simple to solve limitation. Also as part of my evaluation I find it an interesting signal that there has been no comment from Airtable in this 5 year old thread with 51 replies virtually all of which are begging for this feature.

Microsoft Lists has “Enforce Unique Values” and Column Validation.
As Microsoft has budget and resources, AirTable has to step it up. I love AirTable but Unique Values in any field is important.

I am referring to this: https://www.microsoft.com/en-ca/microsoft-365/microsoft-lists
Take a look at this:
https://sharepointmaven.com/how-to-prevent-duplicate-entries-in-sharepoint-lists-and-libraries-with-enforce-unique-values-feature/

I commented on this thread several months ago with one solution to this problem.
You can create a bot using zzBots that will integrate with Airtable and automatically flag duplicate records.
It will allow you to require a unique value for any field that you choose.

You can install it as a plugin here: https://www.zzbots.com/store/ts8rTRcNMqRiHhSvR

After you have it installed, you can watch the following video for reference to finish setting it up, or chat in for assistance.

In this video, the example of the Email field, and Duplicate field is used. These can be replaced with any Airtable field unique to your use case.

Disclaimer: I am affiliated with zzBots

This feature would REALLY help. I’m making, for instance, a view of “Past participants in Tuesday Morning classes” for a new facilitator who will be emailing all the people in the view. If I could make “email” show only unique addresses, it’d save so much time–for the facilitator, and for me when I once again try and fail to figure out how to do this.
Even if, when filtering, I could choose “where email is unique” from the dropdown.

Just add my +1 to the request for the ability to constrain a field to be unique.

This feels like really fundamental requirement after having used other RDBs for about 35 years (e.g. 4D, FileMaker, Omnis, Microsoft SQL Server).

Whilst I appreciate that the universal concept of Record ID goes some way to addressing this, in my case, I’m successfully using the REST API to access data and respect related data within bases, but BETWEEN bases, for some reason, the Record IDs don’t match (for reasons that I suspect are above my pay grade - though fixing that would also be very nice).

Therefore, to work around this issue, I need to match records up (1:1) between two bases based on textual field values being equal, and uniqueness is therefore a requirement for the avoidance of runtime errors.

In the interim, any comments/help from the community would be very much appreciated.

Oof. Stopped Airtable trial for a customer after realizing there wasn’t any way to prevent the common mistake of adding duplicate records in the base price. When this is added, explanation of what’s been duplicated ideally has to bubble up to the forms experience - like perhaps miniExtensions that @Moe mentions provides? - can’t let garbage start coming in. It certainly seems like a big feature but it’s curious to me that it’s not been added after all this time.

Yet another dealbreaker feature that I have discovered during my short AirTable trial. Disappointing that AirTable admins don't seem to engage with their customers in this forum.