I have a list of serial numbers in an excel file and wanto know how many there is but some numbers are duplicated several times.

How can I filter the list so only one of each number is shown?

go to advanced filter and click the unique records only box

what you need to use is a pivot table

1)first hight light all the cells including the header (make sure there is a header to start of list)
2)then data and click pivot table
3)highlight where data is ie excel and form pivot table.
4) next where is the data if you have selected it already in 1 then the cell ranges will be displayed, if not reslect.
5) where do you want data either in new work sheet or a cell on currant worksheet, select as required.
6) click layout at bottom.
7) the cell header you selected should be on the right hand side of the screen, move this into the row column.
8) move same cell header into the data section and then double click it and then select count. ok.
9) finish.

this should then of counted the number of highlighted cells you can do other things with this as well,

Cheers guys, got there in the end.