Symptoms: When looking at your project list you see projects that have the same project number.
Cause: While having duplicate project numbers may be confusing they are not errors in the system and are merely informational. The reason you end up with duplicates is that the server will give the next number in the list when a project is created. If someone creates a project and does not check it in before someone else creates a project the server will give the next number again since it does not know that the other one was used.
Solution: One way around this is to set project numbers by adding a prefix. You can set this in the Setup tab under the Formatting tab. This example is setting each person their own prefix such as SALES01 or SALES02.