diff --git a/contracts/StorageContracts.sol b/contracts/StorageContracts.sol
index 5240b74..8643b4f 100644
--- a/contracts/StorageContracts.sol
+++ b/contracts/StorageContracts.sol
@@ -16,6 +16,7 @@ contract StorageContracts {
     uint proofTimeout; // proof has to be submitted before this
     uint proofMarker; // indicates when a proof is required
     mapping(uint => bool) proofReceived; // whether proof for block was received
+    mapping(uint => bool) proofMissing; // whether proof for block was missing
     uint missingProofs;
   }
 
@@ -221,6 +222,8 @@ contract StorageContracts {
       isProofRequired(contractId, blocknumber),
       "Proof was not required"
     );
+    require(!c.proofMissing[blocknumber], "Proof already marked as missing");
+    c.proofMissing[blocknumber] = true;
     c.missingProofs += 1;
   }
 }
diff --git a/test/StorageContracts.test.js b/test/StorageContracts.test.js
index 1581948..4c402f6 100644
--- a/test/StorageContracts.test.js
+++ b/test/StorageContracts.test.js
@@ -338,6 +338,16 @@ describe("Storage Contracts", function () {
         contracts.markProofAsMissing(id, blocknumber)
       ).to.be.revertedWith("Proof was not required")
     })
+
+    it("does not mark proof as missing twice", async function () {
+      await mineUntilProofIsRequired(id)
+      let blocknumber = await minedBlockNumber()
+      await mineUntilProofTimeout()
+      await contracts.markProofAsMissing(id, blocknumber)
+      await expect(
+        contracts.markProofAsMissing(id, blocknumber)
+      ).to.be.revertedWith("Proof already marked as missing")
+    })
   })
 })