diff --git a/SmartContracts/ProofOfExistence.sol b/SmartContracts/ProofOfExistence.sol
index ee4f8ed179974516b26899a0ee2d770172504486..7b22d85cc179abf8692bf97057dc0cf101862d73 100644
--- a/SmartContracts/ProofOfExistence.sol
+++ b/SmartContracts/ProofOfExistence.sol
@@ -9,7 +9,8 @@ contract ProofOfExistence {
 
 
     function stamp(uint256 ots, uint256 file_hash) public {
-        hashstore[ots] = Dato({blockNumber: block.number, hash: file_hash});
+        if ( hashstore[ots] == 0 )
+            hashstore[ots] = Dato({blockNumber: block.number, hash: file_hash});
     }
 
     function verify(uint256 ots, uint256 file_hash) public view returns(bool){
@@ -26,4 +27,4 @@ contract ProofOfExistence {
         Dato memory dato = hashstore[ots];
         return dato.blockNumber;
     }
-}
\ No newline at end of file
+}