This is usually due to the misalignment of your header cells with your table cells. The Excel export tries to have everything formatted the same as in the report so it will sometimes use two columns for the table cells and merge them so it can align the columns to the header columns. This is problematic when it comes to manipulating, filtering and sorting the spreadsheet.
The best way to avoid this is to create an Excel renderer that doesn't render the header part of the report as described in my answer here.
However, if the cells need to be merged in your report deliberately then you aren't going to be able to do what you want to do using your current report as Excel will duplicate the formatting, including the merged cells.
Probably the only way to get something like what you are after is to create another report that is formatted the way you would like it to be in Excel. In the header of your original report put a text box (or an image with an Excel icon) with an Action on it to open the new, properly formatted, report in Excel, passing across parameters as appropriate. Now the user just need to click on the Action link in the original report to open the more user-friendly report in Excel.
