When is this a problem?
Always.
Why is this a problem?
This practice taxes the system and leaves you without a code, making it much harder to map data when importing to the list.
How to correct:
Exception: If there is no code in the source, you'll have to use a combination of properties. This is rare and should always be a last resort. Work with the IT department to have the source system create a unique code.
More information:
Planual 1.05-04a
Questions on this topic? See Does the list use a combination of properties as a code? in our Model Optimization Checklist forum.