Make PutTransaction<T>() idempotent

CHANGES.md

@@ -69,6 +69,8 @@ To be released.
     if a preloading process suddenly gets shutdown.  [[#417]]
  -  `FileStore` and `LiteDBStore` became to guarantee atomicity of storing
     transactions. [[#413]]
+ -  `IStore.PutTransaction<T>()` became to do nothing when it takes
+    the `Transaction<T>` more than once.  [[#413]]
 
 ### Bug fixes
 

Libplanet/Store/IStore.cs

@@ -77,6 +77,13 @@ public interface IStore
         Transaction<T> GetTransaction<T>(TxId txid)
             where T : IAction, new();
 
+        /// <summary>
+        /// Puts a given <see cref="Transaction{T}"/> to the store.  If the same transaction
+        /// already exists in the store it does nothing.
+        /// </summary>
+        /// <param name="tx">A transaction to put into the store.</param>
+        /// <typeparam name="T">An <see cref="IAction"/> type.  It should match
+        /// to <see cref="Transaction{T}"/>'s type parameter.</typeparam>
         void PutTransaction<T>(Transaction<T> tx)
             where T : IAction, new();
 

Libplanet/Store/LiteDBStore.cs

@@ -271,14 +271,29 @@ public override void PutTransaction<T>(Transaction<T> tx)
             string fileId = TxFileId(tx.Id);
             string filename = tx.Id.ToHex();
             byte[] txBytes = tx.ToBencodex(true);
-            _txLock.EnterWriteLock();
+            _txLock.EnterUpgradeableReadLock();
             try
             {
-                UploadFile(fileId, filename, txBytes);
+                LiteFileInfo file = _db.FileStorage.FindById(fileId);
+                if (file is LiteFileInfo)
+                {
+                    // No-op if already exists.
+                    return;
+                }
+
+                _txLock.EnterWriteLock();
+                try
+                {
+                    UploadFile(fileId, filename, txBytes);
+                }
+                finally
+                {
+                    _txLock.ExitWriteLock();
+                }
             }
             finally
             {
-                _txLock.ExitWriteLock();
+                _txLock.ExitUpgradeableReadLock();
             }
         }