title: Why does setuptools sometimes delete and then re-install the exact same egg?
body: <p>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:</p>
  
  <pre><code>['-v', '-m', '-f', 'R:/OPTIONS/Stephen/python_eggs', 'mypkg==1.0_r2009_03_12']
  </code></pre>
  
  <p>While running the easy_install command this was observed:</p>
  
  <pre><code>Searching for mypkg==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\mypkg-1.0_r2009_03_12-py2.4-win32.egg
  Copying mypkg-1.0_r2009_03_12-py2.4-win32.egg to d:\devtools\python24\lib\site-packages
  </code></pre>
  
  <p>What causes this? Why is it that some times the egg is removed and re-installed, and on other occasions the egg is preserved? I've seen it happen a few times on my own PC but I'm not sure how to consistently re-produce the behavior.</p>
  
  <p>I'm using setuptools 0.6c9</p>