The feature is cool, but ...err, if track-by works like then-by, I think that would be kind of abuse.

http://vuejs.org/guide/list.html#track-by, according to this piece, track-by should be considered as identifier rather than comparer.

If you really need a then-by, refer to this issue: #2591

As for the warning info, I've no idea why it appears.
I can't figure out any duplication in your fiddle.

update:

I got why warning comes out.
track-by can not retrieve data from children objects, it works just like id = value[trackByKey]
so you got two duplicated undefined

I think both of your proposals would be a good enhancement.

@Twiknight Duplication happens because the track-by identifier is used as a key. Here is the excerpt from the code:

id = trackByKey ? trackByKey === '$index' ? index : value[trackByKey] : key || value;

As you can see, id is derived by using value[trackByKey], which results in undefined and results in duplicate. What I propose is the following:

id = trackByKey ? trackByKey === '$index' ? index : getPath(value, trackByKey) : key || value;

So the key or path will be valid with getPath.

I'll look into then-by, as I haven't heard of it before.

@dragantl I think I mistook your proposal, I thought you were trying to make track-by a comparer.
And now I got it.

I totally agree with you.

BTW, I mentioned then-by as a generic feature, not referring to a certain directive

This makes sense, I'd welcome a PR if anyone wants to take a crack at it.

I'll make a PR

@dragantl oops, I was just working on it. Thanks for the offer tho!