S4143 — Map values should not be replaced unconditionally

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".

towns.Item(x) = "London"
towns.Item(x) = "Chicago";  // 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.