Failed to load the css-edit-groups package

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[Enter steps to reproduce:]

1. ...
2. ...

**Atom**: 1.27.2 x64
**Electron**: 1.7.15
**OS**: Mac OS X 10.13.2
**Thrown From**: [css-edit-groups](https://github.com/ekonstantinidis/css-edit-groups) package 0.4.2

### Stack Trace

Failed to load the css-edit-groups package

```
At Duplicate key 'cmd-l'

Error: Duplicate key 'cmd-l'
at Object.detectDuplicateKeys (:39851:19)
at addProperty (:171344:42)
at Array.reduce (native)
at Obj (:171336:34)
at transformNode (:171412:18)
at Value (:171293:18)
at transformNode (:171412:18)
at addProperty (:171343:21)
at Array.reduce (native)
at Obj (:171336:34)
at transformNode (:171412:18)
at Value (:171293:18)
at transformNode (:171412:18)
at Block (:171290:18)
at transformNode (:171412:18)
at Object.parse (:171418:22)
at parseObject (:39600:29)
at parseContentsSync (:39626:22)
at Object.readFileSync (:39738:20)
at keymaps.getKeymapPaths.map (:73198:20)
at Array.map (native)
at Package.loadKeymaps (:73196:50)
at measure (:72877:20)
at Package.measure (:72832:25)
at Package.load (:72873:16)
at PackageManager.loadAvailablePackage (:25886:16)
at config.transact (:25799:20)
at Config.transact (:22504:20)
at PackageManager.loadPackages (:25797:23)
at loadStatePromise.loadState.then (:1073:27)
at :null:null

```

### Commands

```
```

### Non-Core Packages

```
box-edit 0.1.3
css-edit-groups 0.4.2
diagrams 0.28.4
edit-in-new-tab 0.10.1
goto-last-edit 0.3.3
Hydrogen 2.5.0
language-lua-wow 0.2.0
language-mediawiki 2.2.0
language-toc-wow 0.2.0
paredit 1.0.0
scss-media-queries 0.4.0
sequence-diagram 0.0.2
table-editor 0.2.2
themed-settings 1.0.1
wow-spell-tooltips 0.4.6
```

Start by reproducing the package load failure in Atom 1.27.2 on the reported environment and inspect the package's keymap configuration for the reported duplicate `cmd-l` binding. Done means `css-edit-groups` loads without the duplicate-key error; the issue provides no file or test path to begin from.

Written by the indexing model from the issue text.