diff --git a/src/error.rs b/src/error.rs
index 3e013a6..9a7f0f6 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -148,3 +148,27 @@ impl IntoResponse for AppError {
 
 /// Convenience result alias used throughout the codebase.
 pub type AppResult<T> = Result<T, AppError>;
+
+/// True if `error` is a Postgres unique/primary-key constraint violation.
+pub fn is_unique_violation(error: &sqlx::Error) -> bool {
+    matches!(error, sqlx::Error::Database(db_err) if db_err.is_unique_violation())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_conflict_status_and_code() {
+        let err = AppError::Conflict("Email".into());
+        let (status, code) = err.status_and_code();
+        assert_eq!(status, StatusCode::CONFLICT);
+        assert_eq!(code, "CONFLICT");
+    }
+
+    #[test]
+    fn test_is_unique_violation_non_db_error() {
+        let err = sqlx::Error::RowNotFound;
+        assert!(!is_unique_violation(&err));
+    }
+}
diff --git a/src/routes/auth.rs b/src/routes/auth.rs
index e7cd18f..b2f02d4 100644
--- a/src/routes/auth.rs
+++ b/src/routes/auth.rs
@@ -62,7 +62,14 @@ pub async fn register(
     .bind(req.full_name.trim())
     .bind(&req.stellar_address)
     .fetch_one(&state.pool)
-    .await?;
+    .await
+    .map_err(|e| {
+        if crate::error::is_unique_violation(&e) {
+            AppError::Conflict("Email".into())
+        } else {
+            AppError::Database(e)
+        }
+    })?;
 
     let user = User::from(row);
 
diff --git a/src/services/batch.rs b/src/services/batch.rs
index 943d493..8c4b91b 100644
--- a/src/services/batch.rs
+++ b/src/services/batch.rs
@@ -1,5 +1,5 @@
 use crate::{
-    error::{AppError, AppResult},
+    error::{is_unique_violation, AppError, AppResult},
     models::{
         batch_payment::{BatchPaymentResult, SendBatchPaymentRequest},
         transaction::TransactionStatus,
@@ -17,12 +17,6 @@ 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
 /// at all":