Prevents Bounty from being claimed twice (#1374)

* signing prefix added

* Minor improvement

* Tests changed

* Successfully tested

* Minor improvements

* Minor improvements

* Revert "Dangling commas are now required. (#1359)"

This reverts commit a6889776f46adca374b6ebf014aa7b0038112a9d.

* updates

* fixes #1356

* Removed extra semicolon.

(cherry picked from commit c87433e0c242f922d37d28710ced32584b561234)

contracts/drafts/BreakInvariantBounty.sol

@@ -47,6 +47,7 @@ contract BreakInvariantBounty is PullPayment, Ownable {
    * @param target contract
    */
   function claim(Target target) public {
+    require(!_claimed);
     address researcher = _researchers[target];
     require(researcher != address(0));
     // Check Target contract invariants

test/drafts/BreakInvariantBounty.test.js

@@ -92,6 +92,10 @@ contract('BreakInvariantBounty', function ([_, owner, researcher, anyone, nonTar
             it('no longer accepts rewards', async function () {
               await assertRevert(ethSendTransaction({ from: owner, to: this.bounty.address, value: reward }));
             });
+
+            it('reverts when reclaimed', async function () {
+              await assertRevert(this.bounty.claim(this.target.address, { from: researcher }));
+            });
           });
         });
       });