First make sure that you have the records you want to update in your current found set. This usually means you need to do a show all records before importing.
If that's not the issue, then check your match fields and make sure that the data truly matches. If the data is text, even a single invisible character in the Excel cell or the FileMaker field could keep the data from matching. (And make sure you have designated the correct match fields of course...)
I just added a "Show all records" step to the import script. When it runs tomorrow morning I'll find out. Fingers crossed.
Well this morning there were significantly less dupes. Only 6 instead of 300.
So you'll need to check the data in those 6 to see if you can figure out why they didn't match when the others did.
The script imports with two match fields (verified), both are whole numbers. I checked the source file and the data matches exactly. I checked inside fmp at the dupes and they match. I exported 1 file that had two dupes as tab delim text. I ran diff on them in the shell and they are exact matches. At this point I'm considering throwing in the towel and creating a kill all dupes script.
Any time I get unexpected behavior when sorting, finding or using go to related records that I can't otherwise explain and correct, I take the precaution of re-indexing the fields involved to see if that corrects the issue. You might try re-indexing these two match fields and see what happens.
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".