No matching distribution found when same package is listed with different environment markers

[AndreyMZ]

Environment

• pip version: 18.1
• Python version: 3.7.1
• OS: Debian GNU/Linux 9 (stretch)

Details: https://github.com/docker-library/python/blob/39c500cc8aefcb67a76d518d789441ef85fc771f/3.7/stretch/Dockerfile

Description

pip does not properly work when the same package is listed in a requirements file multiple times with different environment markers.

How to Reproduce

1. Create file requirements.txt:

    mysqlclient ; sys_platform == 'linux'
    mysqlclient ; sys_platform == 'win32' --only-binary=mysqlclient
   

2. Create file Dockerfile:

    FROM python
    COPY ./requirements.txt ./
    RUN pip install -r requirements.txt
   

3. Build docker image:

    docker build .
   

Output (actual behaviour)

C:\bug>docker build .
...
Step 3/3 : RUN pip install -r requirements.txt
 ---> Running in 8f67f72f26d0
Ignoring mysqlclient: markers 'sys_platform == "win32"' don't match your environment
Collecting mysqlclient (from -r requirements.txt (line 1))
  Could not find a version that satisfies the requirement mysqlclient (from -r requirements.txt (line 1)) (from versions: )
No matching distribution found for mysqlclient (from -r requirements.txt (line 1))
The command '/bin/sh -c pip install -r requirements.txt' returned a non-zero code: 1

Expected behavior

Package mysqlclient is successfully installed.

[chrahunt]

The cause appears to be here, where we accumulate options that may be specified multiple times from the requirements.

This is probably also related to #5795.