Same field in two different fragments is returned multiple times by `collectFields`

### What happened?
For this query:
```
fragment TaskID on Task {
remoteId: id
remoteType: __typename
}

fragment Task on Task {
title
remoteId: id
completed
}

query {
queryTask {
...TaskID
...Task
}
}
```
the selection set returned by `collectFields` on `queryTask` contains `remoteId: id` field twice. Probably related to #1300, but seems like a different issue because aliases are used and the fragment is on the same type.

### What did you expect?
I expect that the `remoteId: id` field should be returned only once in the selection set returned by `collectFields`.

### Minimal graphql.schema and models to reproduce
```
type Task {
id: ID!
title: String!
completed: Boolean!
}
```

### versions
- `gqlgen version`? v0.12.2
- `go version`? go1.14 linux/amd64
- dep or go modules? go modules