wallet, rpc: Disallow import of unused() if key already exists

diff --git a/src/wallet/rpc/backup.cpp b/src/wallet/rpc/backup.cpp
index 4b87ba23..cb417a6e 100644
--- a/src/wallet/rpc/backup.cpp
+++ b/src/wallet/rpc/backup.cpp
@@ -265,6 +265,23 @@ static UniValue ProcessDescriptorImport(CWallet& wallet, const UniValue& data, c
                }
             }
 
+            // If this is an unused(KEY) descriptor, check that the wallet doesn't already have other descriptors with this key
+            if (!parsed_desc->HasScripts()) {
+                // Unused descriptors must contain a single key.
+                // Earlier checks will have enforced that this key is either a private key when private keys are enabled,
+                // or that this key is a public key when private keys are disabled.
+                // If we can retrieve the corresponding private key from the wallet, then this key is already in the wallet
+                // and we should not import it.
+                std::set<CPubKey> pubkeys;
+                std::set<CExtPubKey> extpubs;
+                parsed_desc->GetPubKeys(pubkeys, extpubs);
+                std::transform(extpubs.begin(), extpubs.end(), std::inserter(pubkeys, pubkeys.begin()), [](const CExtPubKey& xpub) { return xpub.pubkey; });
+                CHECK_NONFATAL(pubkeys.size() == 1);
+                if (wallet.GetKey(pubkeys.begin()->GetID())) {
+                    throw JSONRPCError(RPC_WALLET_ERROR, "Cannot import an unused() descriptor when its private key is already in the wallet");
+                }
+            }
+
             WalletDescriptor w_desc(std::move(parsed_desc), timestamp, range_start, range_end, next_index);
 
             // Add descriptor to the wallet
diff --git a/test/functional/wallet_importdescriptors.py b/test/functional/wallet_importdescriptors.py
index c22bf103..23bb0cf6 100755
--- a/test/functional/wallet_importdescriptors.py
+++ b/test/functional/wallet_importdescriptors.py
@@ -88,6 +88,22 @@ class ImportDescriptorsTest(BitcoinTestFramework):
         assert_equal(hdkeys[0]["xpub"], xpub)
         wallet.unloadwallet()
 
+    def test_import_unused_key_existing(self):
+        self.log.info("Test import of unused(KEY) with existing KEY")
+        self.nodes[0].createwallet(wallet_name="import_existing_unused")
+        wallet = self.nodes[0].get_wallet_rpc("import_existing_unused")
+
+        hdkeys = wallet.gethdkeys(private=True)
+        assert_equal(len(hdkeys), 1)
+        xprv = hdkeys[0]["xprv"]
+
+        self.test_importdesc({"timestamp": "now", "desc": descsum_create(f"unused({xprv})")},
+                             success=False,
+                             error_code=-4,
+                             error_message="Cannot import an unused() descriptor when its private key is already in the wallet",
+                             wallet=wallet)
+        wallet.unloadwallet()
+
     def run_test(self):
         self.log.info('Setting up wallets')
         self.nodes[0].createwallet(wallet_name='w0', disable_private_keys=False)
@@ -846,6 +862,7 @@ class ImportDescriptorsTest(BitcoinTestFramework):
 
 
         self.test_import_unused_key()
+        self.test_import_unused_key_existing()
 
 if __name__ == '__main__':
     ImportDescriptorsTest(__file__).main()