Why is this an issue?
Storing a value inside a collection at a given key or index and then unconditionally overwriting it without reading the initial value is a case of a "dead store".
list[index] = "value 1";
list[index] = "value 2"; // Noncompliant
dictionary.Add(key, "value 1");
dictionary[key] = "value 2"; // Noncompliant
This practice is redundant and will cause confusion for the reader. More importantly, it is often an error and not what the developer intended to do.