I think the problem here is that it's difficult, if not impossible to enforce a unique key on an Application Group -- because CMOD uses multiple tables to keep query performance linear as the number of documents in the AG grows. You might be able to enforce uniqueness of a key within a single table, but I don't think that's what you're looking for.
You might be able to get very close to what you're trying to do by increasing the number of rows per table to a very large number, like 100 to 200 million -- depending on your volume, a large number like this might cover all your documents for the duration they're kept in CMOD -- or at the very least would make sure that your documents have 'mostly unique' key fields.