In importing data directly from SFDC, I have found that 2 characters (ascii 129 and ascii 160) cause errors that are not easy to track back to the source data.  ex.
| Billing City | _Error_1_ | 
| Oakbrook Terrace | Another row has already been processed with this key | 
I found a Billing City of "áSan Francisco" (but the first character looked like a blank in the SFDC UI). Removing the first character fixed the error that was being reported under Oakbrook Terrance. Here is what was inside the Billing City (second row is the ascii number)     S   a   n       F   r   a   n   c   i   s   c   o    160 83  97  110 32  70  114 97  110 99  105 115 99  111   Has anyone else run into particular characters that cause the import to fail?