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Check the note in the link you’ve posted:
You should only register your lookup once. Attempting to register the same lookup class more than once will lead to LookupAlreadyRegistered errors. A common problem related to the LookupAlreadyRegistered error is related to inconsistant import paths in your project. Prior to Django 1.4 the default manage.py allows for importing both with and without the project name (i.e. from myproject.myapp import lookups or from myapp import lookups). This leads to the lookup.py file being imported twice and the registration code executing twice. Thankfully this is no longer the default in Django 1.4. Keeping your import consistant to include the project name (when your app is included inside the project directory) will avoid these errors.