IIPs are sent twice to addressable ports

Given a graph like this:

```
'foo' -> IN[0] Comp
'bar' -> IN[1] Comp
```

IIPs are sent twice due to a bug in `Graph.loadJSON()` method:

```
'foo' is sent to IN[0]
'foo' is sent to IN[1]
'bar' is sent to IN[1]
'bar' is sent to IN[2]
```

This pull request fixes it and adds it to the Graph spec.