- A
S3 event notifications are eventually consistent and may deliver duplicates.
Why wrong: S3 notifications are generally reliable; duplicates are not a known consistency issue.
- B
The Lambda function is configured with a DLQ that causes retries.
Why wrong: A DLQ is for failed messages; successful invocations do not retry.
- C
The Lambda function is idempotent and should handle duplicates.
Why wrong: While idempotency is good practice, the question asks the cause, not the solution.
- D
The S3 bucket has multiple event notifications that trigger the same Lambda function.
Multiple notifications (e.g., for different event types) can cause the same function to be invoked for the same object.
Question 209 of 1,616
Duplicate Lambda Invocations from S3
This DVA-C02 practice question tests your understanding of development with aws services. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A developer is building a serverless application that processes images uploaded to an S3 bucket. The bucket triggers a Lambda function that creates a thumbnail and stores it in another S3 bucket. The developer notices that the Lambda function is invoked multiple times for the same object, causing duplicate thumbnails. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The S3 bucket has multiple event notifications that trigger the same Lambda function.
Option D is correct because the most likely cause of duplicate invocations is that the S3 bucket has multiple event notifications configured for the same event type (e.g., s3:ObjectCreated:*) that all target the same Lambda function. Each notification creates a separate invocation, leading to multiple Lambda executions for the same object upload. This is a common misconfiguration when developers inadvertently add redundant event notifications.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
S3 event notifications are eventually consistent and may deliver duplicates.
Why it's wrong here
S3 notifications are generally reliable; duplicates are not a known consistency issue.
- ✗
The Lambda function is configured with a DLQ that causes retries.
Why it's wrong here
A DLQ is for failed messages; successful invocations do not retry.
- ✗
The Lambda function is idempotent and should handle duplicates.
Why it's wrong here
While idempotency is good practice, the question asks the cause, not the solution.
- ✓
The S3 bucket has multiple event notifications that trigger the same Lambda function.
Why this is correct
Multiple notifications (e.g., for different event types) can cause the same function to be invoked for the same object.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse eventual consistency (which applies to read-after-write for existing objects in S3) with duplicate event delivery, or they incorrectly assume that a DLQ causes retries, when in fact the DLQ only stores failed events after retries are exhausted.
Detailed technical explanation
How to think about this question
S3 event notifications are configured per bucket and can have multiple notification rules for the same event type (e.g., s3:ObjectCreated:*). Each rule triggers independently, so if two rules both match the same upload, the Lambda function is invoked twice. Under the hood, S3 publishes events to the Lambda service via the Invoke API, and each notification rule generates a separate event payload. In real-world scenarios, this often happens when developers copy notification configurations across environments or use AWS CloudFormation with overlapping event selectors.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this DVA-C02 question test?
Development with AWS Services — This question tests Development with AWS Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The S3 bucket has multiple event notifications that trigger the same Lambda function. — Option D is correct because the most likely cause of duplicate invocations is that the S3 bucket has multiple event notifications configured for the same event type (e.g., s3:ObjectCreated:*) that all target the same Lambda function. Each notification creates a separate invocation, leading to multiple Lambda executions for the same object upload. This is a common misconfiguration when developers inadvertently add redundant event notifications.
What should I do if I get this DVA-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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