The DistinctUntilChangedBy operator removes all contiguous elements in the sequence with equal keys. The KeySelector property specifies the member, or set of members, used to test whether elements in the source sequence are equal. Equality is determined by the default EqualityComparer<T> for the type of the selected key.

In other words, after each element is emitted by the result sequence, subsequent elements will be dropped until the value of the key changes.