A track expression specified in the @for loop evaluated to duplicated keys for a given collection, ex.:
@Component({ template: `@for (item of items; track item.value) {{{item.value}}}`,})class TestComponent { items = [{key: 1, value: 'a'}, {key: 2, value: 'b'}, {key: 3, value: 'a'}];}
In the provided example the item.key tracking expression will find two duplicate keys a (at index 0 and 2).
Duplicate keys are problematic from the correctness point of view: since the @for loop can't uniquely identify items it might choose DOM nodes corresponding to another item (with the same key) when performing DOM moves or destroy.
There is also performance penalty associated with duplicated keys - internally Angular must use more sophisticated and slower data structures while repeating over collections with duplicated keys.
Fixing the error
Change the tracking expression such that it uniquely identifies an item in a collection. In the discussed example the correct track expression would use the unique key property (item.key):
@Component({ template: `@for (item of items; track item.key) {{{item.value}}}`,})class TestComponent { items = [{key: 1, value: 'a'}, {key: 2, value: 'b'}, {key: 3, value: 'a'}];}