Kotlin Pub/Sub sample doesn't mention the need to "debounce"

https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/main/spring-cloud-gcp-samples/spring-cloud-gcp-kotlin-samples/spring-cloud-gcp-kotlin-app-sample/src/main/kotlin/com/example/app/PubSubConfiguration.kt

**Describe the bug**
Our Pub/Sub service is really good at making sure that a message is delivered at least once to a subscriber, but messages can be delivered many times, this is non-intuitive and not mentioning it will confuse developers.

There are two possible fixes:
1. Mention it in the comments that you may get the same message many times.
2. Better would be to have some mechanism to debounce (recognize & ignore duplicate messages) so that developers see a way around this.

Start with spring-cloud-gcp-samples/spring-cloud-gcp-kotlin-samples/spring-cloud-gcp-kotlin-app-sample/src/main/kotlin/com/example/app/PubSubConfiguration.kt. Read the Pub/Sub configuration and determine whether the sample should only document at-least-once delivery or also demonstrate duplicate-message handling; done means the chosen behavior is clear to developers and duplicate delivery is addressed without ambiguity.

Written by the indexing model from the issue text.