Lists keep order and allow repeated values. Sets represent membership without duplicate entries. Dictionaries associate keys with values and preserve insertion order, but assigning an existing key replaces its value. A service often needs more than one of these behaviors at once. For example, a dispatch queue can keep the first occurrence of each identifier while a separate set tracks what you've seen. Write down the order and duplicate policy before choosing a container. In this lesson you'll count events, merge settings, group jobs, and build a stable unique list. Each task includes an empty input or repeated key that exposes an incorrect container choice.