A duplicate in Notes is not the same notion as in relational RDBMSs. The only duplicate possible in Notes is of the same document, and that is flagged as a Replication/Save conflict.
So, to answer your question ; yes, there is a way to check for duplicate values in fields, but you have to program it by yourself - there's no easy function to call on.
That said, it's hardly difficult to implement. Just make a hidden view that selects all docs of the same type than the one you're working on. The first column should be the field you wish to check on, sorted and categorized. Then all you need is a Translation Formula in the field that gathers the existing field values using a @DBColumn, and that compares said values to the existing one. If you have a match, you have a duplicate and you need to alert the user.

Pascal.