@append/prependEdge behavior when node already exists

Today, since https://github.com/facebook/relay/pull/3373 got merged, when using `@append/prependEdge` on an already existing node, no node is appended/prepended.

I get that the previous behavior (ie adding a duplicate node) was broken, but I still think that the current behavior is not ideal, shouldn't we instead move the existing node to bottom/top instead of doing nothing?

If you think the current behavior is still sometimes wanted, we could add an additional parameter like:
```
@prependEdge(whenExists: false)
```

Would you be open to a PR that corrects this behavior?

Start by reviewing pull request 3373 and the implementation of @appendEdge and @prependEdge to understand the current duplicate-node behavior. Compare the proposed alternatives—moving an existing node or adding a whenExists parameter—and identify the expected semantics before locating relevant tests. Done means the behavior is agreed and covered by tests for existing and new nodes.

Written by the indexing model from the issue text.