Asp.net caching strategy/pattern

In my WebApi controller I have the following (pseudo) code that receives update notifications from Instagrams real-time API:

[HttpPost]
public void Post(InstagramUpdate instagramUpdate)
{
    var subscriptionId = instagramUpdate.SubscriptionId;
    var lastUpdate = GetLastUpdate(subscriptionId);

    // To avoid breaking my Instagram request limit, do not fetch new images too often.
    if (lastUpdate.AddSeconds(5) < DateTime.UtcNow)
    {
        // More than 5 seconds ago since last update for this subscription. Get new images
        GetNewImagesFromInstagram(subscriptionId);
        UpdateLastUpdate(subscriptionId, DateTime.UtcNow);
    }
}

This won't work very well if I receive two update notifications for the same subscription almost simultaneously, since lastUpdate won't have been updated until after the first request has been processed. And even if I update lastUpdate directly, there is a "lag" because of the round trip to the database.

What would be the best way to tackle this problem? I'm thinking of using some kind of cache (that would be faster than the database roundtrip), but I'm not sure how. Is there some kind of best practices for these kind of things? I'm guessing it's a common problem: "receive notification, do something if something hasn't been done recently..."