Poetry: Package pandas is already present

[folt]

• [x] I am on the latest Poetry version.
• [x] I have searched the issues of this repo and believe that this is not a duplicate.
• [x] If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Ubuntu
• Poetry version: 0.12.12
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/folt/0138d9081ed4d84a009c2af083b6088a

Issue

(.venv) qqq@good:~/work/www$ poetry add pandas -vvv
Using virtualenv: /home/folt/work/www/.venv

[ValueError]          
Package pandas is already present  

Exception trace:
 /home/folt/.poetry/lib/poetry/_vendor/py3.7/cleo/application.py in run() at line 94
   status_code = self.do_run(input_, output_)
 /home/folt/.poetry/lib/poetry/console/application.py in do_run() at line 88
   return super(Application, self).do_run(i, o)
 /home/folt/.poetry/lib/poetry/_vendor/py3.7/cleo/application.py in do_run() at line 197
   status_code = command.run(input_, output_)
 /home/folt/.poetry/lib/poetry/console/commands/command.py in run() at line 77
   return super(BaseCommand, self).run(i, o)
 /home/folt/.poetry/lib/poetry/_vendor/py3.7/cleo/commands/base_command.py in run() at line 146
   status_code = self.execute(input_, output_)
 /home/folt/.poetry/lib/poetry/_vendor/py3.7/cleo/commands/command.py in execute() at line 107
   return self.handle()
 /home/folt/.poetry/lib/poetry/console/commands/add.py in handle() at line 63
   raise ValueError("Package {} is already present".format(name))

I am trying to install pandas package
But poetry gives an error.
Through the pip install pandas command, everything works

[sdispater]

The pandas package is already present in your pyproject.toml file: https://gist.github.com/folt/0138d9081ed4d84a009c2af083b6088a#file-poetry-L57

If you want to install your packages, you must run poetry install

[santiagobasulto]

Shouldn't this be just a warning? (Package already present, skipping).