I just attempted to reproduce this in a simple way, and things seemed to work for me. Can you confirm these steps for me? (I'm using SQL Server instead of Teradata - don't have one of those lying around on my laptop!)
- In a SQL table, insert a single row with two columns: Date and FieldOne...with values 12/1/2011 and "Record One"
- Connect to this table with Tableau, create an incremental extract using the Date column to identify new rows
- Use "Add Data From File" against the exact and point at an Excel workbook with a single row of data. Values: 1/1/2012, "Record Two".
- A report based on the extract now shows two rows, as expected
- Add a second row to the SQL Server table from step 1: Values: 1/2/1012, "Record Three"
- In Tableau, perform an Incremental Refresh
- The report based on the extact now shows three rows (2 from SQL, 1 from Excel), as expected
Based on what you said, I should be missing my 1/1/2012, "Record Two" row from Excel, right?