diff --git a/yarn-project/archiver/src/factory.ts b/yarn-project/archiver/src/factory.ts
index 5aa14b3ca7a0..aaccab4346cd 100644
--- a/yarn-project/archiver/src/factory.ts
+++ b/yarn-project/archiver/src/factory.ts
@@ -173,12 +173,18 @@ export async function createArchiver(
   return archiver;
 }
 
-/** Registers protocol contracts in the archiver store. */
+/** Registers protocol contracts in the archiver store. Idempotent — skips contracts that already exist (e.g. on node restart). */
 export async function registerProtocolContracts(store: KVArchiverDataStore) {
   const blockNumber = 0;
   for (const name of protocolContractNames) {
     const provider = new BundledProtocolContractsProvider();
     const contract = await provider.getProtocolContractArtifact(name);
+
+    // Skip if already registered (happens on node restart with a persisted store).
+    if (await store.getContractClass(contract.contractClass.id)) {
+      continue;
+    }
+
     const contractClassPublic: ContractClassPublic = {
       ...contract.contractClass,
       privateFunctions: [],
diff --git a/yarn-project/archiver/src/store/contract_class_store.ts b/yarn-project/archiver/src/store/contract_class_store.ts
index 36de477aad42..ebe0d6f9400f 100644
--- a/yarn-project/archiver/src/store/contract_class_store.ts
+++ b/yarn-project/archiver/src/store/contract_class_store.ts
@@ -29,11 +29,15 @@ export class ContractClassStore {
     blockNumber: number,
   ): Promise<void> {
     await this.db.transactionAsync(async () => {
-      await this.#contractClasses.setIfNotExists(
-        contractClass.id.toString(),
+      const key = contractClass.id.toString();
+      if (await this.#contractClasses.hasAsync(key)) {
+        throw new Error(`Contract class ${key} already exists, cannot add again at block ${blockNumber}`);
+      }
+      await this.#contractClasses.set(
+        key,
         serializeContractClassPublic({ ...contractClass, l2BlockNumber: blockNumber }),
       );
-      await this.#bytecodeCommitments.setIfNotExists(contractClass.id.toString(), bytecodeCommitment.toBuffer());
+      await this.#bytecodeCommitments.set(key, bytecodeCommitment.toBuffer());
     });
   }
 
diff --git a/yarn-project/archiver/src/store/contract_instance_store.ts b/yarn-project/archiver/src/store/contract_instance_store.ts
index 63ea37ff9bcb..332605240e04 100644
--- a/yarn-project/archiver/src/store/contract_instance_store.ts
+++ b/yarn-project/archiver/src/store/contract_instance_store.ts
@@ -27,11 +27,14 @@ export class ContractInstanceStore {
 
   addContractInstance(contractInstance: ContractInstanceWithAddress, blockNumber: number): Promise<void> {
     return this.db.transactionAsync(async () => {
-      await this.#contractInstances.set(
-        contractInstance.address.toString(),
-        new SerializableContractInstance(contractInstance).toBuffer(),
-      );
-      await this.#contractInstancePublishedAt.set(contractInstance.address.toString(), blockNumber);
+      const key = contractInstance.address.toString();
+      if (await this.#contractInstances.hasAsync(key)) {
+        throw new Error(
+          `Contract instance at ${key} already exists (deployed at block ${await this.#contractInstancePublishedAt.getAsync(key)}), cannot add again at block ${blockNumber}`,
+        );
+      }
+      await this.#contractInstances.set(key, new SerializableContractInstance(contractInstance).toBuffer());
+      await this.#contractInstancePublishedAt.set(key, blockNumber);
     });
   }
 
diff --git a/yarn-project/archiver/src/store/kv_archiver_store.test.ts b/yarn-project/archiver/src/store/kv_archiver_store.test.ts
index 92ae927f0ef3..bea6c6a6e81c 100644
--- a/yarn-project/archiver/src/store/kv_archiver_store.test.ts
+++ b/yarn-project/archiver/src/store/kv_archiver_store.test.ts
@@ -2367,14 +2367,14 @@ describe('KVArchiverDataStore', () => {
       await expect(store.getContractClass(contractClass.id)).resolves.toBeUndefined();
     });
 
-    it('returns contract class if later "deployment" class was deleted', async () => {
-      await store.addContractClasses(
-        [contractClass],
-        [await computePublicBytecodeCommitment(contractClass.packedBytecode)],
-        BlockNumber(blockNum + 1),
-      );
-      await store.deleteContractClasses([contractClass], BlockNumber(blockNum + 1));
-      await expect(store.getContractClass(contractClass.id)).resolves.toMatchObject(contractClass);
+    it('throws if the same contract class is added again', async () => {
+      await expect(
+        store.addContractClasses(
+          [contractClass],
+          [await computePublicBytecodeCommitment(contractClass.packedBytecode)],
+          BlockNumber(blockNum + 1),
+        ),
+      ).rejects.toThrow(/already exists/);
     });
 
     it('returns undefined if contract class is not found', async () => {
@@ -3431,22 +3431,17 @@ describe('KVArchiverDataStore', () => {
       expect(storedBlock?.archive.root.equals(provisionalBlock.archive.root)).toBe(true);
     });
 
-    it('does not throw when adding the same contract class twice', async () => {
+    it('throws when adding the same contract class twice', async () => {
       const contractClass = await makeContractClassPublic();
       const commitment = await computePublicBytecodeCommitment(contractClass.packedBytecode);
 
-      // Add contract class first time
       await store.addContractClasses([contractClass], [commitment], BlockNumber(1));
-
-      // Add same contract class again - should not throw (uses setIfNotExists)
-      await store.addContractClasses([contractClass], [commitment], BlockNumber(2));
-
-      // Verify contract class exists
-      const retrieved = await store.getContractClass(contractClass.id);
-      expect(retrieved).toBeDefined();
+      await expect(store.addContractClasses([contractClass], [commitment], BlockNumber(2))).rejects.toThrow(
+        /already exists/,
+      );
     });
 
-    it('does not throw when adding the same contract instance twice', async () => {
+    it('throws when adding the same contract instance twice', async () => {
       const contractClass = await makeContractClassPublic();
       await store.addContractClasses(
         [contractClass],
@@ -3462,16 +3457,8 @@ describe('KVArchiverDataStore', () => {
         address: await AztecAddress.random(),
       };
 
-      // Add contract instance first time
       await store.addContractInstances([instance], BlockNumber(1));
-
-      // Add same contract instance again - should not throw (uses set)
-      await store.addContractInstances([instance], BlockNumber(2));
-
-      // Verify instance exists
-      const retrieved = await store.getContractInstance(instance.address, 1000n);
-      expect(retrieved).toBeDefined();
-      expect(retrieved?.address.equals(instance.address)).toBe(true);
+      await expect(store.addContractInstances([instance], BlockNumber(2))).rejects.toThrow(/already exists/);
     });
 
     it('does not duplicate logs when addLogs is called twice with same block', async () => {