diff --git a/backend/canisters/user_index/CHANGELOG.md b/backend/canisters/user_index/CHANGELOG.md
index 7870e2549f..9737a497bd 100644
--- a/backend/canisters/user_index/CHANGELOG.md
+++ b/backend/canisters/user_index/CHANGELOG.md
@@ -5,6 +5,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [unreleased]
 
+### Changed
+
+- Log error if any users have duplicate usernames or principals ([#5645](https://github.com/open-chat-labs/open-chat/pull/5645))
+
 ## [[2.0.1129](https://github.com/open-chat-labs/open-chat/releases/tag/v2.0.1129-user_index)] - 2024-04-05
 
 ### Added
diff --git a/backend/canisters/user_index/impl/src/lifecycle/post_upgrade.rs b/backend/canisters/user_index/impl/src/lifecycle/post_upgrade.rs
index 4edf400e09..0d46d1a7a2 100644
--- a/backend/canisters/user_index/impl/src/lifecycle/post_upgrade.rs
+++ b/backend/canisters/user_index/impl/src/lifecycle/post_upgrade.rs
@@ -1,11 +1,11 @@
 use crate::lifecycle::{init_env, init_state};
 use crate::memory::get_upgrades_memory;
-use crate::Data;
+use crate::{read_state, Data};
 use canister_logger::LogEntry;
 use canister_tracing_macros::trace;
 use ic_cdk_macros::post_upgrade;
 use stable_memory::get_reader;
-use tracing::info;
+use tracing::{error, info};
 use user_index_canister::post_upgrade::Args;
 use utils::cycles::init_cycles_dispenser_client;
 
@@ -24,4 +24,30 @@ fn post_upgrade(args: Args) {
     init_state(env, data, args.wasm_version);
 
     info!(version = %args.wasm_version, "Post-upgrade complete");
+
+    read_state(|state| {
+        let users_with_duplicate_usernames: Vec<_> = state
+            .data
+            .users
+            .users_with_duplicate_usernames
+            .iter()
+            .map(|(u1, u2)| (u1.to_string(), u2.to_string()))
+            .collect();
+
+        let users_with_duplicate_principals: Vec<_> = state
+            .data
+            .users
+            .users_with_duplicate_principals
+            .iter()
+            .map(|(u1, u2)| (u1.to_string(), u2.to_string()))
+            .collect();
+
+        if !users_with_duplicate_usernames.is_empty() {
+            error!(?users_with_duplicate_usernames);
+        }
+
+        if !users_with_duplicate_principals.is_empty() {
+            error!(?users_with_duplicate_principals);
+        }
+    })
 }
diff --git a/backend/canisters/user_index/impl/src/model/user_map.rs b/backend/canisters/user_index/impl/src/model/user_map.rs
index a57c703384..45c6b13daf 100644
--- a/backend/canisters/user_index/impl/src/model/user_map.rs
+++ b/backend/canisters/user_index/impl/src/model/user_map.rs
@@ -18,6 +18,10 @@ pub struct UserMap {
     principal_to_user_id: HashMap<Principal, UserId>,
     #[serde(skip)]
     user_referrals: HashMap<UserId, Vec<UserId>>,
+    #[serde(skip)]
+    pub users_with_duplicate_usernames: Vec<(UserId, UserId)>,
+    #[serde(skip)]
+    pub users_with_duplicate_principals: Vec<(UserId, UserId)>,
     suspected_bots: BTreeSet<UserId>,
     suspended_or_unsuspended_users: BTreeSet<(TimestampMillis, UserId)>,
 }
@@ -330,8 +334,13 @@ impl From<UserMapTrimmed> for UserMap {
                 user_map.user_referrals.entry(referred_by).or_default().push(*user_id);
             }
 
-            user_map.username_to_user_id.insert(&user.username, *user_id);
-            user_map.principal_to_user_id.insert(user.principal, *user_id);
+            if let Some(other_user_id) = user_map.username_to_user_id.insert(&user.username, *user_id) {
+                user_map.users_with_duplicate_usernames.push((*user_id, other_user_id));
+            }
+
+            if let Some(other_user_id) = user_map.principal_to_user_id.insert(user.principal, *user_id) {
+                user_map.users_with_duplicate_principals.push((*user_id, other_user_id));
+            }
         }
 
         user_map