In cases like this it is helpful to view the browser’s developer tools, particularly the network section. You can see the requests the browser actually makes, or doesn’t make due to caching, and the status codes it receives. Look for redirects, retries, etc. Also, proxies, reverse proxies, CDNs, etc can intervene between the browser and your service code.
This situation also points out the importance of idempotency of your service. You may erroneously receive the same request multiple times. What is Idempotency?