public Builder<K, V> put(K key, V value) {
            putAndWarnDuplicateKeys(entries, key, value);
            return this;
        }
Add a key-value pair into the built map. The method will throw
IllegalArgumentException immediately when duplicate keys are
provided.

@return Returns a reference to this object so that method calls can
        be chained together.
aws-java-sdk-core/src/main/java/com/amazonaws/util/ImmutableMapParameter.java