diff --git a/contracts/contracts/core/ProviderRegistry.sol b/contracts/contracts/core/ProviderRegistry.sol
index 3d5c63770..dbea39cd5 100644
--- a/contracts/contracts/core/ProviderRegistry.sol
+++ b/contracts/contracts/core/ProviderRegistry.sol
@@ -310,6 +310,7 @@ contract ProviderRegistry is
         for (uint256 i = 0; i < numKeys; ++i) {
             bytes memory key = blsPublicKeys[i];
             require(key.length == 48, InvalidBLSPublicKeyLength(key.length, 48));
+            require(!blockBuilderBLSKeyToAddress[key], BLSKeyAlreadyRegistered(key));
             blockBuilderBLSKeyToAddress[key] = provider;
         }
         eoaToBlsPubkeys[provider] = blsPublicKeys;
diff --git a/contracts/contracts/interfaces/IProviderRegistry.sol b/contracts/contracts/interfaces/IProviderRegistry.sol
index 089060927..694f38342 100644
--- a/contracts/contracts/interfaces/IProviderRegistry.sol
+++ b/contracts/contracts/interfaces/IProviderRegistry.sol
@@ -60,6 +60,7 @@ interface IProviderRegistry {
     error ProviderCommitmentsPending(address sender, uint256 numPending);
     error StakeTransferFailed(address sender, uint256 amount);
     error ProviderAlreadyRegistered(address sender);
+    error BLSKeyAlreadyRegistered(bytes blsKey);
     error InsufficientStake(uint256 stake, uint256 minStake);
     error InvalidBLSPublicKeyLength(uint256 length, uint256 expectedLength);
     error ProviderNotRegistered(address sender);