Designing idempotent APIs: best practices and implementation strategies?

I'm working on a new service and need to ensure our APIs are idempotent. This is critical for reliable client retries and preventing duplicate actions, especially in payment processing. What are your go-to strategies for designing idempotent endpoints? I'm thinking about using a client-provided `idempotency-key` header, but I'm curious about the best ways to store and validate these keys on the server side. Are there specific database patterns you recommend for tracking these keys and their associated outcomes? Also, how do you handle idempotency across multiple services in a distributed transaction scenario?