If the Issue Key column is set as the first column in Issue Navigator, the resultant Excel file that was generated after exporting to Excel (current fields) shows that there are two issue keys in the Issue Key column.

Steps to reproduce:

1. Administration > Issue Fields > Navigator Columns
2. Move Issue Key as the first column.
3. Go to Issue Navigator.
4. Views > Excel (Current fields)
5. Open up the resultant exported Excel file.
6. Notice that there are two issue keys in the first column.