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 "dead store".
letters.put("a", "Apple");
letters.put("a", "Boy"); // Noncompliant
towns[i] = "London";
towns[i] = "Chicago"; // Noncompliant
At best it is redundant and will confuse the reader, most often it is an error and not what the developer intended to do.