diff --git a/account/src/auth/sign_arb.rs b/account/src/auth/sign_arb.rs
index 0fb998c..7000536 100644
--- a/account/src/auth/sign_arb.rs
+++ b/account/src/auth/sign_arb.rs
@@ -30,8 +30,8 @@ fn wrap_message(msg_bytes: &[u8], signer: Addr) -> Vec<u8> {
 #[cfg(test)]
 mod tests {
     use crate::auth::sign_arb::wrap_message;
+    use crate::auth::util;
     use crate::auth::Authenticator::Secp256K1;
-    use crate::auth::{util, Authenticator};
     use crate::contract::instantiate;
     use crate::msg::InstantiateMsg;
     use base64::{engine::general_purpose, Engine as _};
@@ -122,6 +122,6 @@ mod tests {
             signature: Binary::from(signature_bytes),
         };
 
-        let res = instantiate(deps.as_mut(), env.clone(), info, instantiate_msg).unwrap();
+        instantiate(deps.as_mut(), env.clone(), info, instantiate_msg).unwrap();
     }
 }
diff --git a/account/src/error.rs b/account/src/error.rs
index 0832421..39a02ca 100644
--- a/account/src/error.rs
+++ b/account/src/error.rs
@@ -56,6 +56,9 @@ pub enum ContractError {
 
     #[error("invalid token")]
     InvalidToken,
+
+    #[error("cannot override existing authenticator at index {index}")]
+    OverridingIndex { index: u8 },
 }
 
 pub type ContractResult<T> = Result<T, ContractError>;
diff --git a/account/src/execute.rs b/account/src/execute.rs
index bb6e198..399abda 100644
--- a/account/src/execute.rs
+++ b/account/src/execute.rs
@@ -1,6 +1,7 @@
 use cosmwasm_std::{Addr, Binary, Deps, DepsMut, Env, Event, MessageInfo, Order, Response};
 
 use crate::auth::{AddAuthenticator, Authenticator};
+use crate::error::ContractError::OverridingIndex;
 use crate::{
     auth,
     error::{ContractError, ContractResult},
@@ -118,7 +119,7 @@ pub fn add_auth_method(
             )? {
                 Err(ContractError::InvalidSignature)
             } else {
-                AUTHENTICATORS.save(deps.storage, id, &auth)?;
+                save_authenticator(deps, id, &auth)?;
                 Ok(())
             }
         }
@@ -137,7 +138,7 @@ pub fn add_auth_method(
             )? {
                 Err(ContractError::InvalidSignature)
             } else {
-                AUTHENTICATORS.save(deps.storage, id, &auth)?;
+                save_authenticator(deps, id, &auth)?;
                 Ok(())
             }
         }
@@ -156,7 +157,7 @@ pub fn add_auth_method(
             )? {
                 Err(ContractError::InvalidSignature)
             } else {
-                AUTHENTICATORS.save(deps.storage, id, &auth)?;
+                save_authenticator(deps, id, &auth)?;
                 Ok(())
             }
         }
@@ -176,7 +177,7 @@ pub fn add_auth_method(
             )? {
                 Err(ContractError::InvalidSignature)
             } else {
-                AUTHENTICATORS.save(deps.storage, id, &auth)?;
+                save_authenticator(deps, id, &auth)?;
                 Ok(())
             }
         }
@@ -192,6 +193,19 @@ pub fn add_auth_method(
     )
 }
 
+pub fn save_authenticator(
+    deps: DepsMut,
+    id: u8,
+    authenticator: &Authenticator,
+) -> ContractResult<()> {
+    if AUTHENTICATORS.has(deps.storage, id) {
+        return Err(OverridingIndex { index: id });
+    }
+
+    AUTHENTICATORS.save(deps.storage, id, authenticator)?;
+    Ok(())
+}
+
 pub fn remove_auth_method(
     deps: DepsMut,
     env: Env,
@@ -237,7 +251,7 @@ mod tests {
 
     #[test]
     fn test_before_tx() {
-        let authId = 0;
+        let auth_id = 0;
         let mut deps = mock_dependencies();
         let env = mock_env();
 
@@ -251,13 +265,15 @@ mod tests {
         let sig_arr = general_purpose::STANDARD.decode(signature).unwrap();
 
         // The index of the first authenticator is 0.
-        let credIndex = vec![0u8];
+        let cred_index = vec![0u8];
 
         let mut new_vec = Vec::new();
-        new_vec.extend_from_slice(&credIndex);
+        new_vec.extend_from_slice(&cred_index);
         new_vec.extend_from_slice(&sig_arr);
 
-        AUTHENTICATORS.save(deps.as_mut().storage, authId, &auth);
+        AUTHENTICATORS
+            .save(deps.as_mut().storage, auth_id, &auth)
+            .unwrap();
 
         let sig_bytes = Binary::from(new_vec);
         let tx_bytes = Binary::from(general_purpose::STANDARD.decode("Cp0BCpoBChwvY29zbW9zLmJhbmsudjFiZXRhMS5Nc2dTZW5kEnoKP3hpb24xbTZ2aDIwcHM3NW0ybjZxeHdwandmOGZzM2t4dzc1enN5M3YycnllaGQ5c3BtbnUwcTlyc2g0NnljeRIreGlvbjFlMmZ1d2UzdWhxOHpkOW5ra2s4NzZuYXdyd2R1bGd2NDYwdnpnNxoKCgV1eGlvbhIBMRJTCksKQwodL2Fic3RyYWN0YWNjb3VudC52MS5OaWxQdWJLZXkSIgog3pl1PDD1NqnoBnBk5J0wjYzvUFAkWKGTN2lgHc+PAUcSBAoCCAESBBDgpxIaFHhpb24tbG9jYWwtdGVzdG5ldC0xIAg=").unwrap());