Such duplicates should not be possible, period. This is a long established feature of FileMaker for which there have been no known issues for a very long time. So while I can't say this is impossible, I find it highly unlikely. I suggest looking at other possible causes for your duplicate values. Might you have imported some records from one copy of this file into another?
I'm really glad to hear that it should not happen. It happens enough (maybe 6 times now) that I have created a script to detect duplicate serial numbers. A little more information I neglected to include:
* It's a dual file system (App and Data). No data is stored in the app file. All user IDs and passwords are sync'd between the 2 files. Both files have identical Entity Relationship Diagrams.
* I've never done a data import from another FileMaker file or any other external file.
* One culprit I've considered is that I have uploaded a test version of the App file to the server (app_test.fmp12), logged into it and connected to the Data file. I guess that someone could have been logged into the App.fmp12 version while I was testing. In this case, would the Serial Number be created/tracked by the App file or the Data file? If it's the App file, I could see the possibility to duplicate serial numbers.
The serial number auto-enter field option should be part of the field definition in the data file. The UI file that you use to create new records in the table shouldn't matter. Are you sure that this is really an auto-entered serial number field option and not a value entered by a script? Is there ever a need to reset the next serial value such as a system where the serial sequence is intended to start over each month or each year?
Another possibility is that you are not looking at the correct field. Example: Say you have an invoice system with a related Invoice Data table of line items for each invoice. The InvoiceID field found in the invoice table could be set up as an auto-entered serial number and would always be a unique value in that table, but the match field in Invoice Data would also store these serial numbers, but in this table, the values are not unique as all the Invoice Data records for a given invoice would have the same Serial number value. So if you search for duplicates in the Invoice Data table, you'll find them even though none exist in the Invoice table.
Note: if this auto-enter field option failed to produce unique values, Report an Issue would be flooded with impassioned posts demanding an ASAP update to correct the problem as this would spell disaster for a relational database system like FileMaker. I just don't see any such reports now or in the past.
I'm clueless then on why it's happening. Here is the __pk field properties for one of the 70 tables (and hundreds of instances) in the database. All are structured the same. All are Auto-Enter Serial Number, on creation, prohibit modification. The sequence number is irrelevant to the application so they never get reset. I am definitely looking at the correct field. Each table has a single __pk*******ID field which is the auto-enter serial number field.
With the properties (per the picture) on that field, is it even possible to overwrite the unique value?
I agree that it would be reported en masse if it happened often. If it happens again, what can I send you to check into the issue?