diff --git a/src/checkpoints.cpp b/src/checkpoints.cpp
index 078a4ff93f256..d4b378e60b9a1 100644
--- a/src/checkpoints.cpp
+++ b/src/checkpoints.cpp
@@ -81,4 +81,18 @@ namespace Checkpoints
         }
         return NULL;
     }
+
+    bool HaveCheckpoint(int nHeight)
+    {
+        assert(nHeight >= 0);
+
+        if (!fTestNet && (unsigned int)nHeight < sizeof(LSBCheckpoints)/sizeof(int))
+            return true;
+
+        MapCheckpoints& checkpoints = (fTestNet ? mapCheckpointsTestnet : mapCheckpoints);
+
+        MapCheckpoints::const_iterator i = checkpoints.find(nHeight);
+        if (i == checkpoints.end()) return false;
+        return true;
+    }
 }
diff --git a/src/checkpoints.h b/src/checkpoints.h
index 70e936564c4a5..b8dab3a05f1a8 100644
--- a/src/checkpoints.h
+++ b/src/checkpoints.h
@@ -22,6 +22,9 @@ namespace Checkpoints
 
     // Returns last CBlockIndex* in mapBlockIndex that is a checkpoint
     CBlockIndex* GetLastCheckpoint(const std::map<uint256, CBlockIndex*>& mapBlockIndex);
+
+    // Returns true if we have a checkpoint lock-in for the given height
+    bool HaveCheckpoint(int nHeight);
 }
 
 #endif
diff --git a/src/main.cpp b/src/main.cpp
index 3052cfb8c4083..9f4307cdee0bf 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1322,18 +1322,6 @@ bool CBlock::ConnectBlock(CTxDB& txdb, CBlockIndex* pindex)
     if (!CheckBlock())
         return false;
 
-    // Do not allow blocks that contain transactions which 'overwrite' older transactions,
-    // unless those are already completely spent.
-    // If such overwrites are allowed, coinbases and transactions depending upon those
-    // can be duplicated to remove the ability to spend the first instance -- even after
-    // being sent to another address.
-    // See BIP30 and http://r6.ca/blog/20120206T005236Z.html for more information.
-    // This logic is not necessary for memory pool transactions, as AcceptToMemoryPool
-    // already refuses previously-known transaction id's entirely.
-    // This rule applies to all blocks whose timestamp is after March 15, 2012, 0:00 UTC.
-    int64 nBIP30SwitchTime = 1331769600;
-    bool fEnforceBIP30 = (pindex->nTime > nBIP30SwitchTime);
-
     // BIP16 didn't become active until Apr 1 2012
     int64 nBIP16SwitchTime = 1333238400;
     bool fStrictPayToScriptHash = (pindex->nTime >= nBIP16SwitchTime);
@@ -1348,7 +1336,16 @@ bool CBlock::ConnectBlock(CTxDB& txdb, CBlockIndex* pindex)
     {
         uint256 hashTx = tx.GetHash();
 
-        if (fEnforceBIP30) {
+        // Do not allow blocks that contain transactions which 'overwrite' older transactions,
+        // unless those are already completely spent.
+        // If such overwrites are allowed, coinbases and transactions depending upon those
+        // can be duplicated to remove the ability to spend the first instance -- even after
+        // being sent to another address.
+        // See BIP30 and http://r6.ca/blog/20120206T005236Z.html for more information.
+        // This logic is not necessary for memory pool transactions, as AcceptToMemoryPool
+        // already refuses previously-known transaction id's entirely.
+        // This rule applies to all blocks whose timestamp is after March 15, 2012, 0:00 UTC.
+        if (!Checkpoints::HaveCheckpoint(pindex->nHeight)) {
             CTxIndex txindexOld;
             if (txdb.ReadTxIndex(hashTx, txindexOld)) {
                 BOOST_FOREACH(CDiskTxPos &pos, txindexOld.vSpent)