# After modifying expression, new result doesn't replace old one

After I modify and re-evaluate an expression, the new result doesn't replace the old one. Rather, what happens is that the old result gets pushed down and Mathematica displays the current result at the top. It looks like this:

In[20] := (* expression *)
Out[20] = (* result *)
Out[19] = (* older result, when In[20] was In[19] *)
Out[18] = ...
...etc


I'm pretty sure I messed up my preferences, but I can't revert to defaults, because I have another issure resolved (after trying for some time) and not sure exactly how I did it. Following old

If It Works, Don't Fix It

So I'm reluctant to restore the default settings, the other (fixed) issue being far more important to me.

(Mathematica 9 on Ubuntu 12)

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If you re-run the same cell again, it will replace the old output. So if you are are in the In[9] cell, and then hit ENTER, now Out[9] will go away, and instead you will get a new In[10] and a new Out[10] where Out[10] has replaced Out[9]. If that is not what you are getting, then this will be very strange. Are you sure you are evaluating the same cell again? You can always go back to original setting. See support.wolfram.com/kb/3274 –  Nasser Aug 19 '13 at 15:34
Yes. I modify my working cell and output simply stacks on the old one. I'm using single cell. I'm trying to get it to normal behavior (i.e. to replace content of output cell (to use just two cells - one input and one output, not 21 like in example) –  3141593 Aug 19 '13 at 15:40
I see the behaviour described if I delete an expression cell along with its output and then do undo (Ctrl+Z). Further modification to the expression cell lists the output below it without overwriting. Presumably some of the cell defaults are changed in the deletion and undo. (Win 7 M9.0.1) –  Chris Degnen Aug 19 '13 at 15:41
Have you tried to delete all output (Menu: Cell -> Delete All Output) and rerun the notebook? –  Yves Klett Aug 19 '13 at 15:56
It looks like it has something to do with CellAutoOverwrite -> False but I've failed with setting this False or True to see the deifference. It is easy to find in OptionsInspector. –  Kuba Aug 19 '13 at 16:00

You have probably unchecked the notebook option OutputAutoOverwrite. It should normally look like this: