Make confirmation idempotent and add re-confirmation coverage

src/Core/API/API.Specs/Features/Confirmation.feature

@@ -10,6 +10,15 @@ Feature: User Account Confirmation
       Then the response has HTTP status 200
       And the response JSON should have "message" containing "is confirmed"
 
+   Scenario: Re-confirming an already verified account remains successful
+      Given the API is running
+      And I have registered a new account
+      And I have a valid confirmation token for my account
+      When I submit a confirmation request with the valid token
+      And I submit the same confirmation request again
+      Then the response has HTTP status 200
+      And the response JSON should have "message" containing "is confirmed"
+
    Scenario: Confirmation fails with invalid token
       Given the API is running
       When I submit a confirmation request with an invalid token

src/Core/API/API.Specs/Steps/AuthSteps.cs

@@ -599,6 +599,25 @@ public class AuthSteps(ScenarioContext scenario)
         scenario[ResponseBodyKey] = responseBody;
     }
 
+    [When("I submit the same confirmation request again")]
+    public async Task WhenISubmitTheSameConfirmationRequestAgain()
+    {
+        var client = GetClient();
+        var token = scenario.TryGetValue<string>("confirmationToken", out var t)
+            ? t
+            : "valid-token";
+
+        var requestMessage = new HttpRequestMessage(
+            HttpMethod.Post,
+            $"/api/auth/confirm?token={Uri.EscapeDataString(token)}"
+        );
+
+        var response = await client.SendAsync(requestMessage);
+        var responseBody = await response.Content.ReadAsStringAsync();
+        scenario[ResponseKey] = response;
+        scenario[ResponseBodyKey] = responseBody;
+    }
+
     [When("I submit a confirmation request with a malformed token")]
     public async Task WhenISubmitAConfirmationRequestWithAMalformedToken()
     {

src/Core/Infrastructure/Infrastructure.Repository/Auth/AuthRepository.cs

@@ -2,6 +2,7 @@ using System.Data;
 using System.Data.Common;
 using Domain.Entities;
 using Infrastructure.Repository.Sql;
+using Microsoft.Data.SqlClient;
 
 namespace Infrastructure.Repository.Auth;
 
@@ -132,6 +133,12 @@ public class AuthRepository(ISqlConnectionFactory connectionFactory)
             return null;
         }
 
+        // Idempotency: if already verified, treat as successful confirmation.
+        if (await IsUserVerifiedAsync(userAccountId))
+        {
+            return user;
+        }
+
         await using var connection = await CreateConnection();
         await using var command = connection.CreateCommand();
         command.CommandText = "USP_CreateUserVerification";
@@ -139,12 +146,39 @@ public class AuthRepository(ISqlConnectionFactory connectionFactory)
 
         AddParameter(command, "@UserAccountID_", userAccountId);
 
-        await command.ExecuteNonQueryAsync();
+        try
+        {
+            await command.ExecuteNonQueryAsync();
+        }
+        catch (SqlException ex) when (IsDuplicateVerificationViolation(ex))
+        {
+            // A concurrent request verified this user first. Keep behavior idempotent.
+        }
 
         // Fetch and return the updated user
         return await GetUserByIdAsync(userAccountId);
     }
 
+    private async Task<bool> IsUserVerifiedAsync(Guid userAccountId)
+    {
+        await using var connection = await CreateConnection();
+        await using var command = connection.CreateCommand();
+        command.CommandText =
+            "SELECT TOP 1 1 FROM dbo.UserVerification WHERE UserAccountID = @UserAccountID";
+        command.CommandType = CommandType.Text;
+
+        AddParameter(command, "@UserAccountID", userAccountId);
+
+        var result = await command.ExecuteScalarAsync();
+        return result != null && result != DBNull.Value;
+    }
+
+    private static bool IsDuplicateVerificationViolation(SqlException ex)
+    {
+        // 2601/2627 are duplicate key violations in SQL Server.
+        return ex.Number == 2601 || ex.Number == 2627;
+    }
+
 
     /// <summary>
     /// Maps a data reader row to a UserAccount entity.