Hi Anahous,
Just by way of an example...

Say I wanted the data I enter into Sheet3 to automatically also appear
in the corresponding cell on Sheet1. Then I would use the following
formula in A1 on Sheet1...

=IF(Sheet3!A1="","",Sheet3!A1)

Then I would fill this formula across the required number of columns
and down enough rows so that it will accept the increasing data being
entered on Sheet1.

The formula of course can be started off in any cell on Sheet1 so that,
for example, if you started it in C10 on Sheet1 then that cell will
automatically display the contents of Sheet3!A1.

You do have to be careful with the way you manipulate the data on the
sheet that it is originally entered on (Sheet3 in this example) so that
you don't end up with #Ref! error appearing on the duplicate sheet
(Sheet1 in this example). Avoid things such as copy and paste (unless
it's Paste Special: Values), cell dragging and dropping, deleting cells
(Clear their contents then sort instead).
#Ref! problems are usually solved by selecting the top row with the
formulas then filling down as far as needed (unless, of course, a #Ref!
appears in the top row, then you need top fill up from row 2 first)

Ken Johnson