Duplicate styles create duplicate keys

My generated css file contains this:

```
.btn, .link {
color: blue;
}

.btn {
padding: 10px;
}
```

I expected my object to be like this: 

```
{
link: {color: blue},
btn: {color: blue, padding: 10px}
}
```

However, the preview style gets overridden instead of combined:

```
{
btn: {color: blue},
link: {color: blue},
btn: {padding: 10px}
}
```



