diff --git a/src/db.rs b/src/db.rs
index 2e987fe..f91249c 100644
--- a/src/db.rs
+++ b/src/db.rs
@@ -27,3 +27,10 @@ pub async fn run_migrations(pool: &PgPool) -> Result<()> {
     tracing::info!("Migrations complete");
     Ok(())
 }
+
+/// True if `error` is a PostgreSQL unique/primary-key constraint violation.
+/// Used across routes and services to gracefully handle insert races as `409 Conflict` (#55).
+pub fn is_unique_violation(error: &sqlx::Error) -> bool {
+    matches!(error, sqlx::Error::Database(db_err) if db_err.is_unique_violation())
+}
+
diff --git a/src/routes/auth.rs b/src/routes/auth.rs
index e7cd18f..da0e6d1 100644
--- a/src/routes/auth.rs
+++ b/src/routes/auth.rs
@@ -1,4 +1,5 @@
 use crate::{
+    db::is_unique_violation,
     error::{AppError, AppResult},
     middleware::auth::issue_jwt,
     models::user::{AuthResponse, CreateUserRequest, LoginRequest, User, UserRow},
@@ -31,7 +32,7 @@ pub async fn register(
         return Err(AppError::Validation("full_name is required".into()));
     }
 
-    // Check uniqueness.
+    // Fast-path uniqueness check to avoid unnecessary bcrypt hashing cost.
     let exists: bool = sqlx::query_scalar("SELECT EXISTS(SELECT 1 FROM users WHERE email = $1)")
         .bind(&email)
         .fetch_one(&state.pool)
@@ -47,7 +48,7 @@ pub async fn register(
             AppError::Internal(anyhow::anyhow!("bcrypt error: {e}"))
         })?;
 
-    // Insert.
+    // Insert with unique-violation protection against concurrent registration races (#55).
     let id = Uuid::new_v4();
     let row = sqlx::query_as::<_, UserRow>(
         r#"
@@ -62,7 +63,14 @@ pub async fn register(
     .bind(req.full_name.trim())
     .bind(&req.stellar_address)
     .fetch_one(&state.pool)
-    .await?;
+    .await
+    .map_err(|e| {
+        if is_unique_violation(&e) {
+            AppError::Conflict("Email".into())
+        } else {
+            e.into()
+        }
+    })?;
 
     let user = User::from(row);
 
@@ -122,3 +130,109 @@ pub async fn login(
         "data": AuthResponse { token, user }
     })))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn is_unique_violation_helper_identifies_non_db_errors_as_false() {
+        let err = sqlx::Error::RowNotFound;
+        assert!(!is_unique_violation(&err));
+    }
+}
+
+/// End-to-end database tests for registration unique-constraint race condition (#55).
+/// Run with `cargo test -- --ignored` against a real database instance.
+#[cfg(test)]
+mod db_tests {
+    use super::*;
+    use crate::config::Config;
+    use sqlx::postgres::PgPoolOptions;
+
+    async fn test_pool() -> PgPool {
+        let database_url = std::env::var("DATABASE_URL")
+            .expect("DATABASE_URL must be set to run this integration test");
+        let pool = PgPoolOptions::new()
+            .max_connections(5)
+            .connect(&database_url)
+            .await
+            .expect("failed to connect to DATABASE_URL");
+        sqlx::migrate!("./migrations")
+            .run(&pool)
+            .await
+            .expect("failed to run migrations");
+        pool
+    }
+
+    async fn cleanup_user_by_email(pool: &PgPool, email: &str) {
+        let _ = sqlx::query("DELETE FROM users WHERE email = $1")
+            .bind(email)
+            .execute(pool)
+            .await;
+    }
+
+    #[tokio::test]
+    #[ignore]
+    async fn concurrent_register_calls_for_same_email_returns_one_success_and_one_conflict() {
+        let pool = test_pool().await;
+        let email = format!("test-race-{}@example.test", Uuid::new_v4());
+
+        let config = Config::from_env().unwrap_or_else(|_| Config {
+            app_env: "test".into(),
+            host: "127.0.0.1".into(),
+            port: 8080,
+            database_url: std::env::var("DATABASE_URL").unwrap(),
+            database_max_connections: 5,
+            database_min_connections: 1,
+            database_connect_timeout_secs: 5,
+            jwt_secret: "test_secret_for_registration_tests_12345".into(),
+            jwt_expiry_hours: 24,
+            horizon_url: "https://horizon-testnet.stellar.org".into(),
+            stellar_network_passphrase: "Test SDF Network ; September 2015".into(),
+            soroban_rpc_url: "https://soroban-testnet.stellar.org".into(),
+            allowed_origins: vec!["*".into()],
+            rate_cache_ttl_secs: 30,
+            keeper_enabled: false,
+            keeper_poll_interval_secs: 60,
+            keeper_secret_key: None,
+            escrow_contract_id: None,
+            subscription_contract_id: None,
+            reconciliation_poll_interval_secs: 60,
+            reconciliation_stale_after_secs: 300,
+        });
+
+        let state = Arc::new(AppState {
+            pool: pool.clone(),
+            config,
+            loop_health: crate::BackgroundLoopHealth::default(),
+        });
+
+        let req1 = CreateUserRequest {
+            email: email.clone(),
+            password: "password123".into(),
+            full_name: "User One".into(),
+            stellar_address: None,
+        };
+        let req2 = CreateUserRequest {
+            email: email.clone(),
+            password: "password123".into(),
+            full_name: "User Two".into(),
+            stellar_address: None,
+        };
+
+        let (res1, res2) = tokio::join!(
+            register(State(state.clone()), Json(req1)),
+            register(State(state.clone()), Json(req2))
+        );
+
+        let one_ok = res1.is_ok() || res2.is_ok();
+        let one_conflict = matches!(&res1, Err(AppError::Conflict(_))) || matches!(&res2, Err(AppError::Conflict(_)));
+
+        assert!(one_ok, "Exactly one concurrent registration should succeed");
+        assert!(one_conflict, "The racing registration loser must receive Conflict (409), not a 500 Database error");
+
+        cleanup_user_by_email(&pool, &email).await;
+    }
+}
+
diff --git a/src/services/batch.rs b/src/services/batch.rs
index 943d493..7f0ea12 100644
--- a/src/services/batch.rs
+++ b/src/services/batch.rs
@@ -1,4 +1,5 @@
 use crate::{
+    db::is_unique_violation,
     error::{AppError, AppResult},
     models::{
         batch_payment::{BatchPaymentResult, SendBatchPaymentRequest},
@@ -17,11 +18,6 @@ pub struct BatchPaymentService {
     pool: PgPool,
 }
 
-/// True if `error` is a Postgres unique/primary-key violation — used to
-/// detect losing the race to claim a `batch_submissions` row.
-fn is_unique_violation(error: &sqlx::Error) -> bool {
-    matches!(error, sqlx::Error::Database(db_err) if db_err.is_unique_violation())
-}
 
 /// Decides what a failed submission attempt means for transaction status
 /// (#30). The distinction is "did we get a definitive answer from Horizon
diff --git a/src/services/escrow.rs b/src/services/escrow.rs
index 679dd48..a44ba45 100644
--- a/src/services/escrow.rs
+++ b/src/services/escrow.rs
@@ -1,5 +1,6 @@
 use crate::{
     config::Config,
+    db::is_unique_violation,
     error::{AppError, AppResult},
     models::{
         escrow::{CreateEscrowRequest, Escrow, EscrowActionRequest, EscrowActor, EscrowRow, EscrowStatus},
@@ -32,17 +33,11 @@ impl EscrowAction {
     }
 }
 
-/// True if `error` is a Postgres unique/primary-key violation — used to
-/// detect a duplicate `onchain_escrow_id` (#56), mirroring the same pattern
-/// in `services::batch::is_unique_violation`.
-fn is_unique_violation(error: &sqlx::Error) -> bool {
-    matches!(error, sqlx::Error::Database(db_err) if db_err.is_unique_violation())
-}
-
 pub struct EscrowService {
     pool: PgPool,
 }
 
+
 impl EscrowService {
     pub fn new(pool: PgPool) -> Self {
         Self { pool }
diff --git a/src/services/subscription.rs b/src/services/subscription.rs
index 465a519..4af1999 100644
--- a/src/services/subscription.rs
+++ b/src/services/subscription.rs
@@ -1,5 +1,6 @@
 use crate::{
     config::Config,
+    db::is_unique_violation,
     error::{AppError, AppResult},
     models::{
         subscription::{
@@ -34,17 +35,11 @@ pub struct KeeperRunSummary {
     pub considered: usize,
 }
 
-/// True if `error` is a Postgres unique/primary-key violation — used to
-/// detect a duplicate `onchain_subscription_id` (#56), mirroring the same
-/// pattern in `services::batch::is_unique_violation`.
-fn is_unique_violation(error: &sqlx::Error) -> bool {
-    matches!(error, sqlx::Error::Database(db_err) if db_err.is_unique_violation())
-}
-
 pub struct SubscriptionService {
     pool: PgPool,
 }
 
+
 impl SubscriptionService {
     pub fn new(pool: PgPool) -> Self {
         Self { pool }