• VSCode Version: 1.18.1
• OS Version: macOS High Sierra 10.13.1

Steps to Reproduce:

1. Have two identical vsix files of an extension, but with different version tags. I made two test ones here: (https://github.com/hilleer/vscode-install-cli-test-vsixs)[https://github.com/hilleer/vscode-install-cli-test-vsixs]
2. Install the latest (version 1.1.0 from the example ones`) from within vscode or from the command line.
3. Reload / check that the extension is installed (1.1.0)
4. Install the old version (version 1.0.0 from the example ones) from the command line (`code --install-extension ).
5. Reload vscode, and check the version of the extension - still the version of the first (newest) installed version.

Reproduces without extensions: Yes/No