Directives must appear exactly once on a field

Mayby bug.

Try to merge twice 
```
type User {
  id: ID! @unique
  fullname: String
}

type User {
  id: ID! @unique
  email: String
}
```

Got:
```
schema type User {
  id: ID! **@unique @unique**
  fullname: String
}
```
Error: The field `id` specifies a directive more than once. Directives must appear exactly once on a field.
