Import: reject duplicate reading targets in the CSV wizard

Mapping two Reading columns onto one meter staged two readings per row at the
same timestamp. (meter_id, time) is the reading key, so the commit failed with
a raw EF change-tracker error that named neither the column nor the meter.

- Wizard Validate() rejects duplicate Reading targets, naming the columns and
  the meter. Reading-only: Delivery/TankLevel stage events, not readings.
- CommitAsync() re-validates. Previously only Preview did, so a mapping edited
  after a dry run reached the database unchecked.
- ImportService.GuardDuplicateReadings() backstops the same case for the API
  and any other non-wizard caller, reporting meter + date.

Claude-Session: https://claude.ai/code/session_01V6joyergfvVLFEizH1hJLd

src/Infrastructure/Import/ImportService.cs

@@ -18,6 +18,7 @@ public sealed class ImportService(MeterVaultDbContext db, NormalizationService n
         StagedImport staged, string? sourceName, string? mappingJson, CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(staged);
+        GuardDuplicateReadings(staged);
 
         await using var tx = await _db.Database.BeginTransactionAsync(cancellationToken).ConfigureAwait(false);
 
@@ -89,6 +90,31 @@ public sealed class ImportService(MeterVaultDbContext db, NormalizationService n
         await tx.CommitAsync(cancellationToken).ConfigureAwait(false);
     }
 
+    /// <summary>
+    /// A reading is keyed by (meter, time), so a staged set holding two rows for one meter at one
+    /// timestamp cannot be written. Usually a mapping that points two source columns at one meter —
+    /// report it in those terms instead of letting EF surface a change-tracker error.
+    /// </summary>
+    private static void GuardDuplicateReadings(StagedImport staged)
+    {
+        var duplicates = staged.Readings
+            .GroupBy(r => (r.MeterId, r.Time))
+            .Where(g => g.Count() > 1)
+            .ToList();
+
+        if (duplicates.Count == 0)
+        {
+            return;
+        }
+
+        var sample = string.Join("; ", duplicates.Take(3)
+            .Select(g => $"meter {g.Key.MeterId} at {g.Key.Time:yyyy-MM-dd}"));
+
+        throw new InvalidOperationException(
+            $"{duplicates.Count} duplicate reading(s): the same meter is written twice at the same " +
+            $"timestamp ({sample}). Check that no two mapped columns target the same meter.");
+    }
+
     private static IEnumerable<int> AffectedMeters(StagedImport staged) =>
         staged.Readings.Select(r => r.MeterId)
             .Concat(staged.Events.Select(e => e.MeterId))