Duplicate periods fields are not merged correctly

periods properties of output may contain:
```js
[
  [ [ 5 ], [ 1 ] ],
  [ [ 5 ], [ 3, 4, 5 ] ]
]
``` 
but it should be merged like:
```js
[
  [ [ 5 ], [ 1, 3, 4, 5 ] ]
]
``` 