Remove redundant word from exception message. (#4039)

Author: sklirg

src/Elasticsearch.Net/Utf8Json/Internal/ByteArrayStringHashTable.cs

@@ -53,15 +53,15 @@ public void Add(string key, T value)
         {
             if (!TryAddInternal(Encoding.UTF8.GetBytes(key), value))
             {
-                throw new ArgumentException("Key was already exists. Key:" + key);
+                throw new ArgumentException("Key already exists. Key:" + key);
             }
         }
 
         public void Add(byte[] key, T value)
         {
             if (!TryAddInternal(key, value))
             {
-                throw new ArgumentException("Key was already exists. Key:" + Encoding.UTF8.GetString(key));
+                throw new ArgumentException("Key already exists. Key:" + Encoding.UTF8.GetString(key));
             }
         }