Ensure that finishing a contract can only be done once

contracts/Storage.sol

@@ -9,6 +9,8 @@ contract Storage is Contracts, Proofs, Stakes {
 
   uint private stakeAmount;
 
+  mapping(bytes32=>bool) private finished;
+
   constructor(IERC20 token, uint _stakeAmount) Stakes(token) {
     stakeAmount = _stakeAmount;
   }
@@ -56,7 +58,9 @@ contract Storage is Contracts, Proofs, Stakes {
 
   function finishContract(bytes32 id) public {
     require(block.number > proofEnd(id), "Contract has not ended yet");
+    require(!finished[id], "Contract already finished");
     _unlockStake(host(id));
+    finished[id] = true;
   }
 
   function duration(bytes32 contractId) public view returns (uint) {

test/Storage.test.js

@@ -101,6 +101,17 @@ describe("Storage", function () {
           storage.finishContract(id)
         ).to.be.revertedWith("Contract has not ended yet")
       })
+
+      it("can only be done once", async function () {
+        const end = await storage.proofEnd(id)
+        while (await minedBlockNumber() < end) {
+          await mineBlock()
+        }
+        await storage.finishContract(id)
+        await expect(
+          storage.finishContract(id)
+        ).to.be.revertedWith("Contract already finished")
+      })
     })
   })