I'd double check the invisible characters that may or may not be spaces in these two fields to be sure. One invisible character different is all it takes here.
If you are absolutely, positively sure that records with identical fields aren't being found, you might try rebuilding the index on this field to see if that helps.
To rebuild the index of a single field,
- open Manage | Database | Fields and double click the field
- Use either the storage tab or the storage options button to turn off indexing.
- Exit Manage | Database, then return and turn indexing back on.
You can also rebuild all your file's indexes by importing all the data into an empty copy (clone) of your file.
If you have FileMaker 11, you can use Advanced Recovery options to rebuild your file's indexes:
- With the file closed, select Recover from the File Menu.
- Select "Use advanced Options"
- Select only: "Copy File Blocks as-is" and "Rebuild Field Indexes Now".