Uploading a file with a name that already exists asks the user if they want to overwrite.

This is not perfect (it doesn't check against the real filesystem but the current list in the browser which may be stale) but it is better than nothing.

IPython/html/static/tree/js/notebooklist.js

@@ -430,7 +430,29 define([
                     that.notebook_path,
                     filename
                 );
-                $.ajax(url, settings);
+                
+                var exists = false;
+                $.each(that.element.find('.list_item:not(.new-file)'), function(k,v){
+                    if ($(v).data('name') === filename) { exists = true; return false; }
+                });
+                if (exists) {
+                    dialog.modal({
+                        title : "Replace file",
+                        body : 'There is already a file named ' + filename + ', do you want to replace it?',
+                        buttons : {
+                            Overwrite : {
+                                class: "btn-danger",
+                                click: function() { $.ajax(url, settings); }
+                            },
+                            Cancel : {
+                                click: function() { item.remove(); }
+                            }
+                        }
+                    });
+                } else {
+                    $.ajax(url, settings);
+                }
+                
                 return false;
             });
         var cancel_button = $('<button/>').text("Cancel")