Limit entry of field to values in lookup table or entering doubles

I have some columns that ar filled by looking up a value in the entry array or lookup table. I would like to prevent entering new values (non existing) that are added to the lookup table (which happens now) or are added to the lookup array (which also happens now). In some cases I would also like to enforce a valid entry, rather than allowing blanks.
Is this possible through a function (or otherwise)?

And - is there a way to prevent entering doubles as a new entry in a table?

Hi @joost_mineur,

If you have an example of your setup, we can be more specific with replies.

For Select Lists and Lookup Columns, there is a toggle that either allows new items to be added or prevents it. You can see that here:

Formulas can be used for Select List options and filters can be included to only display the results you want from a table.

Thank you - I had overlooked the option to not allow new values. I will create a sample of my project later on to address some specific questions about not allowing blanks or duplicates - I tried using some formulas, but they seem to obstruct the list function.