Debugging scenarioHard~12 minProduction / 9 of 22

Interview question

A client retry created duplicate records in production. How would you investigate and prevent it?

Duplicate resource created after a client retry

Investigate and fix an API where client retries create duplicate orders, payments, or records.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

I first identify what was duplicated, how many records are affected, and whether there were external side effects like payments or messages. Then I check logs, timestamps, client retry behavior, and whether the API had idempotency keys or uniqueness constraints. I would contain the issue before cleanup. Then I would plan safe data repair.

Full answer below

Practice focus

idempotency, unique constraints, retry behavior, request logging, side effects, and data repair planning

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Investigate and fix an API where client retries create duplicate orders, payments, or records. It matters in real backend work because the team needs observability, ownership, and rollback room to recover without guessing. The practical angle is idempotency, unique constraints, retry behavior, request logging, side effects, and data repair planning, tied to a concrete production decision.
I first identify what was duplicated, how many records are affected, and whether there were external side effects like payments or messages. Then I check logs, timestamps, client retry behavior, and whether the API had idempotency keys or uniqueness constraints. I would contain the issue before cleanup. Then I would plan safe data repair.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
This often happens when the server commits the operation but the client times out before receiving the response. The client retries and the API treats it as a new request. I prevent that with idempotency keys, unique business constraints, or both. The trade-off is extra storage and logic, but it is much safer than assuming network calls happen exactly once.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production, duplicate creation is a correctness incident. I separate containment, customer impact, data repair, external side effects, and prevention. For prevention, I use transactional idempotency records, request-body hashes, unique constraints, and clear retry semantics. I also review client timeout settings, server timeouts, retry policies, and monitoring so duplicates are caught early. In production I would separate mitigation from root cause.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Deleting duplicate rows without understanding side effects
Blaming only the client
Using in-memory duplicate checks
Relying on disabled buttons
Not adding unique constraints where appropriate

Red flagsCandidateSigns the approach is fragileSignals that an interviewer may treat as risky.Open details

Details

Cannot explain timeout-after-commit
Does not know idempotency key patterns
Ignores payments/messages
Does not plan data repair

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

How would you choose an idempotency key?
What if the same key has a different body?
How do database constraints help?
How do external providers affect this?

Continue the section

Next in Production: Thread Pool Starvation

Step 10 of 22

Thread pool starvation symptoms in an ASP.NET Core API

Opening drill

Debugging scenarioHard~12 minProduction / 9 of 22

Interview question

A client retry created duplicate records in production. How would you investigate and prevent it?

Duplicate resource created after a client retry

Investigate and fix an API where client retries create duplicate orders, payments, or records.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Full answer below

Practice focus

idempotency, unique constraints, retry behavior, request logging, side effects, and data repair planning

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Investigate and fix an API where client retries create duplicate orders, payments, or records. It matters in real backend work because the team needs observability, ownership, and rollback room to recover without guessing. The practical angle is idempotency, unique constraints, retry behavior, request logging, side effects, and data repair planning, tied to a concrete production decision.
I first identify what was duplicated, how many records are affected, and whether there were external side effects like payments or messages. Then I check logs, timestamps, client retry behavior, and whether the API had idempotency keys or uniqueness constraints. I would contain the issue before cleanup. Then I would plan safe data repair.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
This often happens when the server commits the operation but the client times out before receiving the response. The client retries and the API treats it as a new request. I prevent that with idempotency keys, unique business constraints, or both. The trade-off is extra storage and logic, but it is much safer than assuming network calls happen exactly once.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production, duplicate creation is a correctness incident. I separate containment, customer impact, data repair, external side effects, and prevention. For prevention, I use transactional idempotency records, request-body hashes, unique constraints, and clear retry semantics. I also review client timeout settings, server timeouts, retry policies, and monitoring so duplicates are caught early. In production I would separate mitigation from root cause.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Deleting duplicate rows without understanding side effects
Blaming only the client
Using in-memory duplicate checks
Relying on disabled buttons
Not adding unique constraints where appropriate

Red flagsCandidateSigns the approach is fragileSignals that an interviewer may treat as risky.Open details

Details

Cannot explain timeout-after-commit
Does not know idempotency key patterns
Ignores payments/messages
Does not plan data repair

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

How would you choose an idempotency key?
What if the same key has a different body?
How do database constraints help?
How do external providers affect this?

Continue the section

Next in Production: Thread Pool Starvation

Step 10 of 22

Thread pool starvation symptoms in an ASP.NET Core API

A client retry created duplicate records in production. How would you investigate and prevent it?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue the section

A client retry created duplicate records in production. How would you investigate and prevent it?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue the section