Version

3.0.5

Reproduction link

https://codesandbox.io/s/vigorous-booth-0sq44?file=/src/App.vue

Steps to reproduce

1、click repoduction link 2、see console 3、click change button 4、see console

What is expected?

should warn duplicate keys

What is actually happening?

no warning appears

relate #3078

The diff algorithm is able to handle a lot of fast cases without the need for creating a full key map - and in such cases Vue can correctly update the list even with duplicated keys.

e.g. a b c a -> a a can work even if there are duplicated keys because the head and tail remains the same.

The current duplicate key check only warns when a full key map is needed and the duplicated key leads to ambiguous behavior.

Checking duplicated keys on every update is wasteful even in development so it's best to avoid it.