Update --force-fieldtrial-params comment to be a little more clear.

chrome/common/variations/variations_util.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/common/variations/variations_util.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <vector>
 
 #include "base/feature_list.h"
 #include "base/metrics/field_trial.h"
 #include "base/strings/string_split.h"
+#include "base/strings/stringprintf.h"
 #include "chrome/common/variations/fieldtrial_testing_config.h"
 #include "components/variations/variations_associated_data.h"
 #include "net/base/escape.h"
 
 namespace chrome_variations {
 
 namespace {
 
 std::string EscapeValue(const std::string& value) {
   return net::UnescapeURLComponent(value, net::UnescapeRule::URL_SPECIAL_CHARS);
 }
 
 } // namespace
 
 bool AssociateParamsFromString(const std::string& varations_string) {
   // Format: Trial1.Group1:k1/v1/k2/v2,Trial2.Group2:k1/v1/k2/v2
+  std::set<std::pair<std::string, std::string>> trial_groups;
   for (const base::StringPiece& experiment_group : base::SplitStringPiece(
            varations_string, ",",
            base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL)) {
     std::vector<base::StringPiece> experiment = base::SplitStringPiece(
         experiment_group, ":", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
     if (experiment.size() != 2) {
       DLOG(ERROR) << "Experiment and params should be separated by ':'";
       return false;
     }
 
     std::vector<std::string> group_parts = base::SplitString(
         experiment[0], ".", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
     if (group_parts.size() != 2) {
       DLOG(ERROR) << "Study and group name should be separated by '.'";
       return false;
     }
 
     std::vector<std::string> key_values = base::SplitString(
         experiment[1], "/", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
     if (key_values.size() % 2 != 0) {
       DLOG(ERROR) << "Param name and param value should be separated by '/'";
       return false;
     }
-
+    std::string trial = EscapeValue(group_parts[0]);
+    std::string group = EscapeValue(group_parts[1]);
+    auto trial_group = std::make_pair(trial, group);
+    if (trial_groups.find(trial_group) != trial_groups.end()) {
+      DLOG(ERROR) << base::StringPrintf(
+          "A (study, group) pair listed more than once. (%s, %s)",
+          trial.c_str(), group.c_str());
+      return false;
+    }
+    trial_groups.insert(trial_group);
     std::map<std::string, std::string> params;
     for (size_t i = 0; i < key_values.size(); i += 2) {
       std::string key = EscapeValue(key_values[i]);
       std::string value = EscapeValue(key_values[i + 1]);
       params[key] = value;
     }
-    std::string trial = EscapeValue(group_parts[0]);
-    std::string group = EscapeValue(group_parts[1]);
     variations::AssociateVariationParams(trial, group, params);
   }
   return true;
 }
 
 void AssociateParamsFromFieldTrialConfig(const FieldTrialTestingConfig& config,
                                          base::FeatureList* feature_list) {
   for (size_t i = 0; i < config.groups_size; ++i) {
     const FieldTrialTestingGroup& group = config.groups[i];
     if (group.params_size != 0) {
@@ skipping 19 matching lines @@
           base::FeatureList::OVERRIDE_DISABLE_FEATURE, trial);
     }
   }
 }
 
 void AssociateDefaultFieldTrialConfig(base::FeatureList* feature_list) {
   AssociateParamsFromFieldTrialConfig(kFieldTrialConfig, feature_list);
 }
 
 }  // namespace chrome_variations