Record all requests made to web service?

I built a web service that takes requests from SITE1 and then plugs the values into SITE2. We're running into an issue where occasionally SITE1 will send the request twice within a very short timeframe (within 30 seconds - 5 minutes). If the initial request completes fast enough, I have safe guards that prevent against duplicate records being entered. However, if a second request comes in before the first has completed, the web service completes both requests concurrently.

I need to figure out if there is a way to store a global temporary list of requests so that I can compare against this list prior to inserting the values.

My web service operates on a pretty simple level, it is simple http://ift.tt/1JiV1mR. All I would need to do is store the incoming ID in a global list until the request has completed successfully.

What options are available to me?