diff --git a/src/main/scala/com/fluency03/blockchain/core/Block.scala b/src/main/scala/com/fluency03/blockchain/core/Block.scala
index 30f76e8..19c0415 100644
--- a/src/main/scala/com/fluency03/blockchain/core/Block.scala
+++ b/src/main/scala/com/fluency03/blockchain/core/Block.scala
@@ -6,6 +6,8 @@ import com.fluency03.blockchain.core.Transaction.createCoinbaseTx
 import org.json4s.native.JsonMethods.{compact, render}
 import org.json4s.{Extraction, JValue}
 
+import scala.collection.mutable
+
 /**
  * Block on the chain.
  * @param header Header of current Block
@@ -37,6 +39,8 @@ case class Block(header: BlockHeader, transactions: Seq[Transaction], hash: Stri
 
   def hasValidHeaderHash: Boolean = hash == header.hash
 
+  def allTransactionsValid(uTxOs: mutable.Map[Outpoint, TxOut]): Boolean = transactions.forall(tx => tx.isValid(uTxOs))
+
   def toJson: JValue = Extraction.decompose(this)
 
   override def toString: String = compact(render(toJson))
diff --git a/src/main/scala/com/fluency03/blockchain/core/Transaction.scala b/src/main/scala/com/fluency03/blockchain/core/Transaction.scala
index d3260d1..5baae69 100644
--- a/src/main/scala/com/fluency03/blockchain/core/Transaction.scala
+++ b/src/main/scala/com/fluency03/blockchain/core/Transaction.scala
@@ -134,12 +134,17 @@ object Transaction {
       .map(t => t.txOuts.zipWithIndex.map {
         case (txOut, index) => Outpoint(t.id, index) -> txOut
       }.toMap)
-      .reduce { _ ++ _ }
+      .foldLeft(Map.empty[Outpoint, TxOut])(_ ++ _)
 
   def getConsumedUTxOs(transactions: Seq[Transaction]): Map[Outpoint, TxOut] =
     transactions.map(_.txIns)
-      .reduce { _ ++ _ }
+      .foldLeft(Seq.empty[TxIn])(_ ++ _)
       .map(txIn => Outpoint(txIn.previousOut.id, txIn.previousOut.index) -> TxOut("", 0))
       .toMap
 
+  def noDuplicateTxIn(transactions: Seq[Transaction]): Boolean = {
+    val allRefs = transactions.map(_.txIns.map(_.previousOut)).foldLeft(Seq.empty[Outpoint])(_ ++ _)
+    allRefs.distinct.length == allRefs.length
+  }
+
 }
diff --git a/src/test/scala/com/fluency03/blockchain/core/TransactionTest.scala b/src/test/scala/com/fluency03/blockchain/core/TransactionTest.scala
index c8e5c01..0809724 100644
--- a/src/test/scala/com/fluency03/blockchain/core/TransactionTest.scala
+++ b/src/test/scala/com/fluency03/blockchain/core/TransactionTest.scala
@@ -207,4 +207,37 @@ class TransactionTest extends FlatSpec with Matchers {
     signedTx2.isValid(uTxOs) shouldEqual true
   }
 
+  "noDuplicateTxIn" should "detect whether Seq of Transactions contians duplicate TxIns." in {
+    noDuplicateTxIn(Seq.empty[Transaction]) shouldEqual true
+    val tx1: Transaction = Transaction(
+      Seq(TxIn(Outpoint("a", 0), ""),
+        TxIn(Outpoint("b", 1), "")),
+      Seq(TxOut("o", 40)),
+      genesisTimestamp
+    )
+    noDuplicateTxIn(Seq(tx1)) shouldEqual true
+    val tx2: Transaction = Transaction(
+      Seq(TxIn(Outpoint("c", 0), ""),
+        TxIn(Outpoint("d", 1), "")),
+      Seq(TxOut("o", 40)),
+      genesisTimestamp
+    )
+    noDuplicateTxIn(Seq(tx1, tx2)) shouldEqual true
+    val tx3: Transaction = Transaction(
+      Seq(TxIn(Outpoint("e", 0), ""),
+        TxIn(Outpoint("a", 1), "")),
+      Seq(TxOut("o", 40)),
+      genesisTimestamp
+    )
+    noDuplicateTxIn(Seq(tx1, tx2, tx3)) shouldEqual true
+    val tx4: Transaction = Transaction(
+      Seq(TxIn(Outpoint("a", 0), ""),
+        TxIn(Outpoint("f", 1), "")),
+      Seq(TxOut("o", 40)),
+      genesisTimestamp
+    )
+    noDuplicateTxIn(Seq(tx1, tx2, tx3, tx4)) shouldEqual false
+  }
+
+
 }