function addPayee(address payee, uint256 share) external override onlyManager {
    if (_payees.contains(payee)) {
        revert Errors.PAYEE_ALREADY_EXISTS();
    }
    ...
}
diff --git a/test/kuma-protocol/KUMAFeeCollector.t.sol b/test/kuma-protocol/KUMAFeeCollector.t.sol
index f34d9ff..0b3fe46 100644
--- a/test/kuma-protocol/KUMAFeeCollector.t.sol
+++ b/test/kuma-protocol/KUMAFeeCollector.t.sol
@@ -40,6 +40,39 @@ contract KUMAFeeCollectorTest is BaseSetUp {
         );
     }

+    function test_DuplicatePayees() public {
+        address[] memory newPayees = new address[](4);
+        uint256[] memory newShares = new uint256[](4);
+
+        newPayees[0] = vm.addr(10);
+        newPayees[1] = vm.addr(10);
+        newPayees[2] = vm.addr(11);
+        newPayees[3] = vm.addr(12);
+        newShares[0] = 25;
+        newShares[1] = 25;
+        newShares[2] = 25;
+        newShares[3] = 25;
+
+        _KUMAFeeCollector.changePayees(newPayees, newShares);
+
+        // only 3 payees
+        assertEq(_KUMAFeeCollector.getPayees().length, 3);
+        // newPayees[0] and newPayees[1] are identical and both are added as payees[0]
+        address[] memory payees = _KUMAFeeCollector.getPayees();
+        assertEq(payees[0], newPayees[1]);
+        assertEq(payees[1], newPayees[2]);
+        assertEq(payees[2], newPayees[3]);
+
+        uint256 countedTotalShares = 0;
+        for (uint i; i < payees.length; i++) {
+            countedTotalShares += _KUMAFeeCollector.getShare(payees[i]);
+        }
+        // Counted totalShares is 75 (100 - 25)
+        assertEq(countedTotalShares, 75);
+        // Recorded totalShares is 100
+        assertEq(_KUMAFeeCollector.getTotalShares(), 100);
+    }
+
     function test_initialize() public {
         assertEq(address(_KUMAFeeCollector.getKUMAAddressProvider()), address(_KUMAAddressProvider));
         assertEq(_KUMAFeeCollector.getRiskCategory(), _RISK_CATEGORY);
forge test -m test_DuplicatePayees
[] Compiling...
No files changed, compilation skipped

Running 1 test for test/kuma-protocol/KUMAFeeCollector.t.sol:KUMAFeeCollectorTest
[PASS] test_DuplicatePayees() (gas: 259689)
Test result: ok. 1 passed; 0 failed; finished in 7.39ms
diff --git a/src/kuma-protocol/KUMAFeeCollector.sol b/src/kuma-protocol/KUMAFeeCollector.sol
index 402cf71..1a9d86d 100644
--- a/src/kuma-protocol/KUMAFeeCollector.sol
+++ b/src/kuma-protocol/KUMAFeeCollector.sol
@@ -180,7 +180,9 @@ contract KUMAFeeCollector is IKUMAFeeCollector, UUPSUpgradeable, Initializable {
             }

             address payee = newPayees[i];
-            _payees.add(payee);
+            if (!_payees.add(payee)) {
+                revert Errors.PAYEE_ALREADY_EXISTS();
+            }
             _shares[payee] = newShares[i];
             _totalShares += newShares[i];
