IT would seem that the file from which you are importing records already has assigned a serial number to the field in that other copy of the file.
I would think enabling auto-enter options would, however, replace that value with a new value in the imported table so this is a bit surprising.
However, you don't have to include that field when mapping fields for the import and then (if enabled) the auto-entered serial number setting should work.
Here's are some other ways to deal with this:
If you discover wrong serial numbers after importing, you can use Replace Field Contents to assign serial numbers ot he found set of newly imported records and update your serial number settings at the same time.
If you are using FileMaker 12, you can replace your serial number field with a text field that auto-enters Get ( UUID ) in both copies of this file. Then the records will import with a value that is already unique even though it comes from the other copy of the file.
Just ran a quick test and yes, if there is data in the serial number field, that data will be imported instead of replacing it with a new value even if you enable the auto-enter option, but if you exclude the field from the import mapping order, then you get the expected auto-entered serial number.
Hello PhilModJunk -
Thanks so much for your suggestions/recommendations! I appreciate that and I'll try them and see where I get!