[Distutils] Why does setuptools sometimes delete and then re-install the exact same egg?

[Fadhley Salim]

Searching for calyon==1.0-r2009-03-12
Best match: calyon 1.0-r2009-03-12
Processing calyon-1.0_r2009_03_12-py2.4-win32.egg
Removing
d:\devtools\python24\lib\site-packages\calyon-1.0_r2009_03_12-py2.4-win3
2.egg
Copying calyon-1.0_r2009_03_12-py2.4-win32.egg to
d:\devtools\python24\lib\site-packages

What causes this?

I've seen it happen a few times but I'm not sure how to re-produce the
behavior.

I'm trying to install an egg on a computer where an identical egg
already exists. Why does it remove the egg and then re-install it? I'm
calling easy_install from a script with the options:

['-vv', '-m', '-f', 'R:/OPTIONS/Stephen/python_eggs',
'calyon==1.0_r2009_03_12']

I'm using setuptools 0.6c9
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