diff --git a/modules/swagger-codegen/src/main/resources/csharp/Configuration.mustache b/modules/swagger-codegen/src/main/resources/csharp/Configuration.mustache
index 5ce03342baa..d2d7a9ad34b 100644
--- a/modules/swagger-codegen/src/main/resources/csharp/Configuration.mustache
+++ b/modules/swagger-codegen/src/main/resources/csharp/Configuration.mustache
@@ -158,9 +158,24 @@ namespace {{packageName}}.Client
         /// <returns></returns>
         public void AddDefaultHeader(string key, string value)
         {
+            if (_defaultHeaderMap.ContainsKey(key))
+                _defaultHeaderMap.Remove(key);
             _defaultHeaderMap.Add(key, value);
         }
 
+        /// <summary>
+        /// Add Api Key.
+        /// </summary>
+        /// <param name="key">Api Key name.</param>
+        /// <param name="value">Api Key value.</param>
+        /// <returns></returns>
+        public void AddApiKeyPrefix(string key, string value)
+        {
+            if (ApiKeyPrefix.ContainsKey(key))
+                ApiKeyPrefix.Remove(key);
+            ApiKeyPrefix.Add(key, value);
+        }
+
         /// <summary>
         /// Gets or sets the HTTP user agent.
         /// </summary>