An AngularJS $http decorator to filter duplicate requests (now released as library: https://github.com/meanie/angular-duplicate-requests-filter)

duplicateRequestsFilter.js

/**
 * Module definition and dependencies
 */
angular.module('Api.DuplicateRequestsFilter.Decorator', [])

/**
 * Config
 */
.config(function($provide) {

  /**
   * Decorator for the $http service
   */
  $provide.decorator('$http', function($delegate, $q) {

    /**
     * Pending requests and local $http var for natural reference
     */
    var pendingRequests = {};
    var $http = $delegate;

    /**
     * Hash generator
     */
    function hash(str) {
      var h = 0;
      var strlen = str.length;
      if (strlen === 0) {
        return h;
      }
      for (var i = 0, n; i < strlen; ++i) {
        n = str.charCodeAt(i);
        h = ((h << 5) - h) + n;
        h = h & h;
      }
      return h >>> 0;
    }

    /**
     * Helper to generate a unique identifier for a request
     */
    function getRequestIdentifier(config) {
      var str = config.method + config.url;
      if (config.params && typeof config.params === 'object') {
        str += angular.toJson(config.params);
      }
      if (config.data && typeof config.data === 'object') {
        str += angular.toJson(config.data);
      }
      return hash(str);
    }

    /**
     * Modified $http service
     */
    var $duplicateRequestsFilter = function(config) {

      //Ignore for this request?
      if (config.ignoreDuplicateRequest) {
        return $http(config);
      }

      //Get unique request identifier
      var identifier = getRequestIdentifier(config);

      //Check if such a request is pending already
      if (pendingRequests[identifier]) {
        if (config.rejectDuplicateRequest) {
          return $q.reject({
            data: '',
            headers: {},
            status: config.rejectDuplicateStatusCode || 400,
            config: config
          });
        }
        return pendingRequests[identifier];
      }

      //Create promise using $http and make sure it's reset when resolved
      pendingRequests[identifier] = $http(config).finally(function() {
        delete pendingRequests[identifier];
      });

      //Return promise
      return pendingRequests[identifier];
    };

    //Map rest of methods
    Object.keys($http).filter(function(key) {
      return (typeof $http[key] === 'function');
    }).forEach(function(key) {
      $duplicateRequestsFilter[key] = $http[key];
    });

    //Return it
    return $duplicateRequestsFilter;
  });
});

@AdamiecRadek thanks for your comment, I haven't had issues myself with the filter in our production apps, and we also use Authorization headers as well as some additional custom headers, so curious to know at what point it went wrong. Glad you found a solution though.