diff --git a/src/main.cpp b/src/main.cpp
index e5d110d8ef..427e435a90 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1652,6 +1652,16 @@ bool CBlock::CheckBlock() const
         if (!tx.CheckTransaction())
             return DoS(tx.nDoS, error("CheckBlock() : CheckTransaction failed"));
 
+    // Check for duplicate txids. This is caught by ConnectInputs(),
+    // but catching it earlier avoids a potential DoS attack:
+    set<uint256> uniqueTx;
+    BOOST_FOREACH(const CTransaction& tx, vtx)
+    {
+        uniqueTx.insert(tx.GetHash());
+    }
+    if (uniqueTx.size() != vtx.size())
+        return DoS(100, error("CheckBlock() : duplicate transaction"));
+
     unsigned int nSigOps = 0;
     BOOST_FOREACH(const CTransaction& tx, vtx)
     {