When you validate that field you can get the view column as a collection and see if the newly typed value is a member of that collection.

If it is then validation fails and they have to type in another Value

Thank you for your answer. I guess it was something like that. But how will i use it and where will i trigg it when i use xpages validation for fields?

is where I do my field validations.  If this event returns "false" then the document is not saved and the page remains open.

Use SSJS to do the lookup.