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

• OS version and name: MacOS 10.15.5
• Poetry version: 1.1.2
• Link of a Gist with the contents of your pyproject.toml file: N/A

Issue

I have notice a regression compared to poetry 1.0.10 in the following case:

• I add package dependency with a python version constraint ("typing-extensions")
• I add a development dependency having the same sub-dependency with no python version constraint ("black").

The command poetry export give a wrong result with poetry 1.1.2 (it's OK with 1.0.10)

How to reproduce:

poetry new example
cd example
sed -i '/^python = /s/3.8/3.7/' pyproject.toml
poetry add --python="<3.8" typing-extensions
poetry export -f requirements.txt | grep typing
poetry add --dev black
poetry export -f requirements.txt | grep typing

Output for Poetry 1.0.10 (Python 3.8.6):

...
typing-extensions==3.7.4.3; python_version < "3.8" \
...
typing-extensions==3.7.4.3 \

Output for Poetry 1.1.2 (Python 3.8.6):

...
typing-extensions==3.7.4.3; python_version < "3.8" \
...
typing-extensions==3.7.4.3; python_version < "3.8" \