diff --git a/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml b/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml
index 2fca806..e4b825e 100644
--- a/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml
+++ b/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml
@@ -104,5 +104,12 @@
             <tr><th>Importiert am</th><td>@Model.ImportedInvoice.ImportedAt.ToString("dd.MM.yyyy HH:mm")</td></tr>
         </table>
         <div class="alert alert-success mt-2">✔ Rechnung wurde in der Datenbank gespeichert (ID: @Model.ImportedInvoice.Id)</div>
+        @if (Model.IsDuplicate)
+        {
+            <div class="alert alert-warning mt-2">
+                ⚠️ <strong>Duplikat:</strong> Diese Rechnung wurde bereits importiert (ID: @Model.ImportedInvoice!.Id).
+                Es wurde kein neuer Eintrag angelegt.
+            </div>
+        }
     }
 }
\ No newline at end of file
diff --git a/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml.cs b/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml.cs
index 759bb6f..94d1407 100644
--- a/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml.cs
+++ b/DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml.cs
@@ -21,6 +21,7 @@ public class UploadModel(
     public string? ErrorMessage { get; private set; }
     public ZugferdInvoice? ImportedInvoice { get; private set; }
     public string? ResultFilePath { get; private set; }
+    public bool IsDuplicate { get; private set; }
 
     public void OnGet()
     { }
@@ -56,10 +57,14 @@ public class UploadModel(
             if (Result.HasZugferdXml)
             {
                 memStream.Position = 0;
-                ImportedInvoice = await zugferdImportService.ImportAsync(memStream, "Upload");
+                ImportedInvoice = await zugferdImportService.ImportAsync(memStream, "Upload", Result.ZugferdGuidelineId);
 
-                // 3. Result-Package erstellen (nur wenn Import erfolgreich)
-                if (ImportedInvoice is not null)
+                // Duplikat erkennen: vorhandener Eintrag hat ImportedAt von früher
+                if (ImportedInvoice is not null && ImportedInvoice.ImportedAt < DateTime.UtcNow.AddSeconds(-5))
+                    IsDuplicate = true;
+
+                // 3. Result-Package erstellen (nur wenn Import erfolgreich UND kein Duplikat)
+                if (ImportedInvoice is not null && !IsDuplicate)
                 {
                     ResultFilePath = await resultPackageService.CreateResultPackageAsync(
                         originalBytes, PdfFile.FileName, ImportedInvoice);
diff --git a/DXApp.TemplateKitProject/Services/ZugferdImportService.cs b/DXApp.TemplateKitProject/Services/ZugferdImportService.cs
index 57c58c5..e54e882 100644
--- a/DXApp.TemplateKitProject/Services/ZugferdImportService.cs
+++ b/DXApp.TemplateKitProject/Services/ZugferdImportService.cs
@@ -1,5 +1,6 @@
 using DXApp.TemplateKitProject.Data;
 using DXApp.TemplateKitProject.Models;
+using Microsoft.EntityFrameworkCore;
 
 namespace DXApp.TemplateKitProject.Services;
 
@@ -9,7 +10,7 @@ public class ZugferdImportService(
     AppDbContext db,
     ILogger<ZugferdImportService> logger)
 {
-    public async Task<ZugferdInvoice?> ImportAsync(Stream pdfStream, string sourceType)
+    public async Task<ZugferdInvoice?> ImportAsync(Stream pdfStream, string sourceType, string guidelineId = "")
     {
         var xml = extractor.ExtractXml(pdfStream);
 
@@ -20,11 +21,31 @@ public class ZugferdImportService(
         }
 
         var invoice = parser.Parse(xml);
+
+        // Duplikatprüfung
+        var duplicate = await db.ZugferdInvoices.FirstOrDefaultAsync(i =>
+            i.InvoiceNumber == invoice.InvoiceNumber &&
+            i.SellerTaxId == invoice.SellerTaxId);
+
+        if (duplicate is not null)
+        {
+            logger.LogWarning(
+                "Duplikat erkannt: Rechnung '{Number}' von '{Seller}' existiert bereits (ID: {Id}).",
+                invoice.InvoiceNumber, invoice.SellerName, duplicate.Id);
+            return duplicate;
+        }
+
         invoice.SourceType = sourceType;
+        invoice.GuidelineId = guidelineId;
+        invoice.ImportedAt = DateTime.UtcNow;
 
         db.ZugferdInvoices.Add(invoice);
         await db.SaveChangesAsync();
 
+        logger.LogInformation(
+            "Rechnung '{Number}' von '{Seller}' importiert (ID: {Id}).",
+            invoice.InvoiceNumber, invoice.SellerName, invoice.Id);
+
         return invoice;
     }
 }
\ No newline at end of file