使用pip安装python包的时候会多次安装依赖包

比如使用代码：

pip install --index-url https://pypi.douban.com/simple cnvkit
pip install --index-url https://pypi.douban.com/simple HTSeq

上面的会安装:

Installing collected packages: numpy, biopython, future, six, cycler, backports.functools-lru-cache, subprocess32, setuptools, kiwisolver, pytz, python-dateutil, pyparsing, matplotlib, pandas, pyfaidx, pysam, pip, pillow, reportlab, scipy, futures, cnvkit
Successfully installed backports.functools-lru-cache-1.5 biopython-1.72 cnvkit-0.9.3 cycler-0.10.0 future-0.16.0 futures-3.2.0 kiwisolver-1.0.1 matplotlib-2.2.2 numpy-1.14.5 pandas-0.23.3 pillow-5.2.0 pip-10.0.1 pyfaidx-0.5.4.1 pyparsing-2.2.0 pysam-0.14.1 python-dateutil-2.7.3 pytz-2018.5 reportlab-3.4.0 scipy-1.1.0 setuptools-40.0.0 six-1.11.0 subprocess32-3.5.2

下面的会安装；

Installing collected packages: numpy, pysam, HTSeq
Successfully installed HTSeq-0.10.0 numpy-1.14.5 pysam-0.14.1

很明显 pysam-0.14.1 就被安装了两次。

而且我查看目录：

ls -lh $HOME/.local/lib/python2.7/site-packages

也的确看到包对应的文件夹更新了。