diff --git a/Mk0.Software.Bildduplikate/Main.Designer.cs b/Mk0.Software.Bildduplikate/Main.Designer.cs
index 574cbe3..2b1f2c7 100644
--- a/Mk0.Software.Bildduplikate/Main.Designer.cs
+++ b/Mk0.Software.Bildduplikate/Main.Designer.cs
@@ -45,6 +45,7 @@ namespace Mk0.Software.Bildduplikate
             this.richTextBoxDesc2 = new System.Windows.Forms.RichTextBox();
             this.buttonAPISetting = new System.Windows.Forms.Button();
             this.buttonAPILoad = new System.Windows.Forms.Button();
+            this.buttonUndo = new System.Windows.Forms.Button();
             ((System.ComponentModel.ISupportInitialize)(this.pictureBox1)).BeginInit();
             ((System.ComponentModel.ISupportInitialize)(this.pictureBox2)).BeginInit();
             ((System.ComponentModel.ISupportInitialize)(this.splitContainer1)).BeginInit();
@@ -119,7 +120,7 @@ namespace Mk0.Software.Bildduplikate
             // buttonKeinDuplikat
             // 
             this.buttonKeinDuplikat.Anchor = System.Windows.Forms.AnchorStyles.Top;
-            this.buttonKeinDuplikat.Location = new System.Drawing.Point(372, 12);
+            this.buttonKeinDuplikat.Location = new System.Drawing.Point(385, 12);
             this.buttonKeinDuplikat.Name = "buttonKeinDuplikat";
             this.buttonKeinDuplikat.Size = new System.Drawing.Size(217, 23);
             this.buttonKeinDuplikat.TabIndex = 8;
@@ -242,11 +243,23 @@ namespace Mk0.Software.Bildduplikate
             this.buttonAPILoad.UseVisualStyleBackColor = true;
             this.buttonAPILoad.Click += new System.EventHandler(this.ButtonAPILoad_Click);
             // 
+            // buttonUndo
+            // 
+            this.buttonUndo.Anchor = System.Windows.Forms.AnchorStyles.Top;
+            this.buttonUndo.Location = new System.Drawing.Point(608, 12);
+            this.buttonUndo.Name = "buttonUndo";
+            this.buttonUndo.Size = new System.Drawing.Size(75, 23);
+            this.buttonUndo.TabIndex = 15;
+            this.buttonUndo.Text = "undo";
+            this.buttonUndo.UseVisualStyleBackColor = true;
+            this.buttonUndo.Click += new System.EventHandler(this.ButtonUndo_Click);
+            // 
             // Main
             // 
             this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
             this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
             this.ClientSize = new System.Drawing.Size(984, 551);
+            this.Controls.Add(this.buttonUndo);
             this.Controls.Add(this.buttonAPILoad);
             this.Controls.Add(this.buttonAPISetting);
             this.Controls.Add(this.splitContainer2);
@@ -295,6 +308,7 @@ namespace Mk0.Software.Bildduplikate
         private System.Windows.Forms.Button buttonAPILoad;
         private System.Windows.Forms.RichTextBox richTextBoxDesc1;
         private System.Windows.Forms.RichTextBox richTextBoxDesc2;
+        private System.Windows.Forms.Button buttonUndo;
     }
 }
 
diff --git a/Mk0.Software.Bildduplikate/Main.cs b/Mk0.Software.Bildduplikate/Main.cs
index 2ac8ca8..22c32cb 100644
--- a/Mk0.Software.Bildduplikate/Main.cs
+++ b/Mk0.Software.Bildduplikate/Main.cs
@@ -18,6 +18,7 @@ namespace Mk0.Software.Bildduplikate
         private string file;
         private bool api = false;
         private int worst = 0;
+        private int lastStep = 0;
 
         private List<Duplicate> duplicates = new List<Duplicate>();
 
@@ -265,7 +266,7 @@ namespace Mk0.Software.Bildduplikate
                             webClient.Headers[HttpRequestHeader.ContentType] = "application/json";
                             string json = webClient.UploadString(Properties.Settings.Default.ApiUrl + "?isDuplicate", "{\"duplicateId\":" + Duplicates.ElementAt(lineDup).ID + "}");
                             var resObj = new JavaScriptSerializer().Deserialize<APIResponse>(json);
-                            if (resObj.response_code != "200") { MessageBox.Show("Fehler", resObj.response_desc, MessageBoxButtons.OK, MessageBoxIcon.Error); }
+                            if (resObj.response_code != "200") { MessageBox.Show(resObj.response_desc, "Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error); }
                             else
                             {
                                 total--;
@@ -401,12 +402,13 @@ namespace Mk0.Software.Bildduplikate
                     webClient.Headers[HttpRequestHeader.ContentType] = "application/json";
                     string json = webClient.UploadString(Properties.Settings.Default.ApiUrl + "?noDuplicate", "{\"duplicateId\":" + Duplicates.ElementAt(line).ID + "}");
                     var resObj = new JavaScriptSerializer().Deserialize<APIResponse>(json);
-                    if (resObj.response_code != "200") { MessageBox.Show("Fehler", resObj.response_desc, MessageBoxButtons.OK, MessageBoxIcon.Error); }
+                    if (resObj.response_code != "200") { MessageBox.Show(resObj.response_desc, "Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error); }
                     else
                     {
                         total--;
                         line++;
                         LoadDuplicates(line, api);
+                        lastStep = 1;
                     }
                 }
                 catch (Exception ex)
@@ -420,6 +422,7 @@ namespace Mk0.Software.Bildduplikate
                 total--;
                 line++;
                 LoadDuplicates(line, api);
+                lastStep = 0;
             }
         }
 
@@ -438,13 +441,14 @@ namespace Mk0.Software.Bildduplikate
                     webClient.Headers[HttpRequestHeader.ContentType] = "application/json";
                     string json = webClient.UploadString(Properties.Settings.Default.ApiUrl + "?isDuplicate", "{\"duplicateId\":" + Duplicates.ElementAt(line).ID + "}");
                     var resObj = new JavaScriptSerializer().Deserialize<APIResponse>(json);
-                    if (resObj.response_code != "200") { MessageBox.Show("Fehler", resObj.response_desc, MessageBoxButtons.OK, MessageBoxIcon.Error); }
+                    if (resObj.response_code != "200") { MessageBox.Show(resObj.response_desc, "Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error); }
                     else
                     {
                         File.Delete(Duplicates.ElementAt(line).Path1);
                         total--;
                         line++;
                         LoadDuplicates(line, api);
+                        lastStep = 2;
                     }
                 }
                 catch (Exception ex)
@@ -459,6 +463,7 @@ namespace Mk0.Software.Bildduplikate
                 line++;
                 total--;
                 LoadDuplicates(line);
+                lastStep = 0;
             }
         }
 
@@ -477,13 +482,14 @@ namespace Mk0.Software.Bildduplikate
                     webClient.Headers[HttpRequestHeader.ContentType] = "application/json";
                     string json = webClient.UploadString(Properties.Settings.Default.ApiUrl + "?isDuplicate", "{\"duplicateId\":" + Duplicates.ElementAt(line).ID + "}");
                     var resObj = new JavaScriptSerializer().Deserialize<APIResponse>(json);
-                    if (resObj.response_code != "200") { MessageBox.Show("Fehler", resObj.response_desc, MessageBoxButtons.OK, MessageBoxIcon.Error); }
+                    if (resObj.response_code != "200") { MessageBox.Show(resObj.response_desc, "Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error); }
                     else
                     {
                         File.Delete(Duplicates.ElementAt(line).Path2);
                         total--;
                         line++;
                         LoadDuplicates(line, api);
+                        lastStep = 2;
                     }
                 }
                 catch (Exception ex)
@@ -498,6 +504,7 @@ namespace Mk0.Software.Bildduplikate
                 line++;
                 total--;
                 LoadDuplicates(line);
+                lastStep = 0;
             }
         }
 
@@ -604,7 +611,7 @@ namespace Mk0.Software.Bildduplikate
                 webClient.Headers[HttpRequestHeader.ContentType] = "application/json";
                 string json = webClient.UploadString(Properties.Settings.Default.ApiUrl + "?getDuplicates", "{\"amount\":" + Properties.Settings.Default.imagesFromAPI + "}");
                 var resObj = new JavaScriptSerializer().Deserialize<APIResponse>(json);
-                if (resObj.response_code != "200") { MessageBox.Show("Fehler", resObj.response_desc, MessageBoxButtons.OK, MessageBoxIcon.Error); }
+                if (resObj.response_code != "200") { MessageBox.Show(resObj.response_desc, "Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error); }
                 else
                 {
                     Duplicates.Clear();
@@ -623,7 +630,7 @@ namespace Mk0.Software.Bildduplikate
                     webClient2.Headers[HttpRequestHeader.ContentType] = "application/json";
                     string json2 = webClient.UploadString(Properties.Settings.Default.ApiUrl + "?totalDuplicates", "");
                     var resObj2 = new JavaScriptSerializer().Deserialize<APIResponse>(json2);
-                    if (resObj2.response_code != "200") { MessageBox.Show("Fehler", resObj2.response_desc, MessageBoxButtons.OK, MessageBoxIcon.Error); }
+                    if (resObj2.response_code != "200") { MessageBox.Show(resObj2.response_desc, "Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error); }
                     else
                     {
                         total = int.Parse(resObj2.totalDuplicates);
@@ -704,6 +711,48 @@ namespace Mk0.Software.Bildduplikate
             }
         }
 
+        /// <summary>
+        /// Undo last processing step
+        /// </summary>
+        /// <param name="sender"></param>
+        /// <param name="e"></param>
+        private void ButtonUndo_Click(object sender, EventArgs e)
+        {
+            if (lastStep == 1)
+            {
+                //undo mark as non-duplicate
+                try
+                {
+                    WebClient webClient = new WebClient();
+                    webClient.Headers[HttpRequestHeader.ContentType] = "application/json";
+                    string json = webClient.UploadString(Properties.Settings.Default.ApiUrl + "?undoDuplicate", "{\"duplicateId\":" + Duplicates.ElementAt(line-1).ID + "}");
+                    var resObj = new JavaScriptSerializer().Deserialize<APIResponse>(json);
+                    if (resObj.response_code != "200") { MessageBox.Show(resObj.response_desc, "Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error); }
+                    else
+                    {
+                        File.Delete(Duplicates.ElementAt(line).Path2);
+                        total++;
+                        line--;
+                        LoadDuplicates(line, api);
+                        lastStep = 2;
+                    }
+                }
+                catch (Exception ex)
+                {
+                    MessageBox.Show("Fehler bei der Kommunikation mit der API:" + Environment.NewLine + ex.ToString(), "API Fehler", MessageBoxButtons.OK, MessageBoxIcon.Error);
+                }
+            }
+            else if (lastStep == 2)
+            {
+                //undo delete duplicate
+            }
+            else
+            {
+                MessageBox.Show("Keine Aktion die Rückgängig gemacht werden kann. Undo funktioniert grundsätzlich nur bei API Bildvergleichen, nicht bei CSV.", "Nix da", MessageBoxButtons.OK, MessageBoxIcon.Error);
+            }
+            //lastStep = 0;
+        }
+
         /// <summary>
         /// Reloads CSV / API
         /// </summary>