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API Gateway supports this via MultiValueHeaders. Consideration should be given to how best to support multi-value parameters in ions.
https://aws.amazon.com/blogs/compute/support-for-multi-value-parameters-in-amazon-api-gateway/
https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#apigateway-multivalue-headers-and-parameters
Related issues in Pedestal.ion, "Duplicate header support (applicable to the set-cookie header)."