Renaming an output with the same name as an existing output overwrites the existing output

[GoogleCodeExporter]

What steps will reproduce the problem?
1.  Run a spatial or cluster analysis.  Call it fred.
2.  Run another one.
3.  Rename the second output to be fred.
4.  Save the project, close and re-open.

There is only one output called fred.  The first one has been deleted.  

The system needs to trap these cases and raise an exception, also warning
the user when the name is in use (as is the case with randomisations - see
issue #54).

Original issue reported on code.google.com by shawnlaffan on 14 Jan 2010 at 7:48