Allow duplicate labels

Importing the course [English Visual Dictionary](http://www.memrise.com/course/54235/english-visual-dictionary/) fails, because the course contains two fields with the same label:
```
pools: {
  columns: {
    "5": {
      kind: "text",
      typing_disabled: false,
      typing_strict: false,
      label: "Text",
      always_show: false,
      classes: [ ],
      keyboard: "",
      tapping_disabled: false
    },
    "7": {
      kind: "text",
      typing_disabled: false,
      typing_strict: false,
      label: "Text",
      always_show: true,
      classes: [ ],
      keyboard: "",
      tapping_disabled: false
    }
    ...
```

This PR contains a quick fix for the problem.