diff --git a/app/controllers/problems_controller.rb b/app/controllers/problems_controller.rb
--- a/app/controllers/problems_controller.rb
+++ b/app/controllers/problems_controller.rb
@@ -156,6 +156,14 @@
       render :action => 'import' and return
     end
 
+    old_problem = Problem.find_by_name(@problem.name)
+    if old_problem!=nil
+      old_problem.full_name = @problem.full_name
+      @problem = old_problem
+      
+      flash[:notice] = "The test data has been replaced for problem #{@problem.name}"
+    end
+
     @problem.save
     @log = import_log
   end
diff --git a/app/controllers/test_controller.rb b/app/controllers/test_controller.rb
--- a/app/controllers/test_controller.rb
+++ b/app/controllers/test_controller.rb
@@ -101,7 +101,7 @@
     end
     @test_requests = []
     @user.test_requests.each do |ts|
-      if ts.problem.available
+      if ts.problem and ts.problem.available
         @test_requests << ts
       end
     end
diff --git a/app/models/problem.rb b/app/models/problem.rb
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -3,8 +3,9 @@
   belongs_to :description
 
   validates_presence_of :name
+  validates_format_of :name, :with => /^\w+$/
   validates_presence_of :full_name
-
+  
   def self.find_available_problems
     find(:all, :conditions => {:available => true}, :order => "date_added DESC")
   end
diff --git a/app/views/graders/list.html.haml b/app/views/graders/list.html.haml
--- a/app/views/graders/list.html.haml
+++ b/app/views/graders/list.html.haml
@@ -4,6 +4,9 @@
 
 %h1 Grader information
 
+= link_to '[Refresh]', :action => 'list'
+%br/
+
 .submitbox
   .item
     Grader control:
diff --git a/lib/testdata_importer.rb b/lib/testdata_importer.rb
--- a/lib/testdata_importer.rb
+++ b/lib/testdata_importer.rb
@@ -42,7 +42,7 @@
     elsif ext=='.tar'
       cmd = "tar -xvf #{testdata_filename} -C #{extract_dir}"
     elsif ext=='.zip'
-      cmd = "unzip #{testdata_filename} -d #{extract_dir}"
+      cmd = "unzip -o #{testdata_filename} -d #{extract_dir}"
     else
       return nil
     end