diff --git a/mod/workshop/exsubmission.php b/mod/workshop/exsubmission.php
index 486c27814ee..3a29bdf3b03 100644
--- a/mod/workshop/exsubmission.php
+++ b/mod/workshop/exsubmission.php
@@ -46,6 +46,14 @@ $workshop = $DB->get_record('workshop', array('id' => $cm->instance), '*', MUST_
 $workshop = new workshop($workshop, $cm, $course);
 
 $PAGE->set_url($workshop->exsubmission_url($id), array('edit' => $edit));
+$PAGE->set_title($workshop->name);
+$PAGE->set_heading($course->fullname);
+if ($edit) {
+    $PAGE->navbar->add(get_string('exampleediting', 'workshop'));
+} else {
+    $PAGE->navbar->add(get_string('example', 'workshop'));
+}
+$output = $PAGE->get_renderer('mod_workshop');
 
 if ($id) { // example is specified
     $example = $workshop->get_example_by_id($id);
@@ -95,6 +103,15 @@ if ($id and $assess and $canassess) {
     if (!$asid) {
         $asid = $workshop->add_allocation($example, $USER->id, 0);
     }
+    if ($asid == workshop::ALLOCATION_EXISTS) {
+        // the training assessment of the example was not found but the allocation already
+        // exists. this probably means that the user is the author of the reference assessment.
+        echo $output->header();
+        echo $output->box(get_string('assessmentreferenceconflict', 'workshop'));
+        echo $output->continue_button($workshop->view_url());
+        echo $output->footer();
+        die();
+    }
     redirect($workshop->exassess_url($asid));
 }
 
@@ -167,16 +184,7 @@ if ($edit and $canmanage) {
     }
 }
 
-$PAGE->set_title($workshop->name);
-$PAGE->set_heading($course->fullname);
-if ($edit) {
-    $PAGE->navbar->add(get_string('exampleediting', 'workshop'));
-} else {
-    $PAGE->navbar->add(get_string('example', 'workshop'));
-}
-
 // Output starts here
-$output = $PAGE->get_renderer('mod_workshop');
 echo $output->header();
 echo $output->heading(format_string($workshop->name), 2);
 
diff --git a/mod/workshop/lang/en/workshop.php b/mod/workshop/lang/en/workshop.php
index 2302ebb18f9..f51361fc875 100644
--- a/mod/workshop/lang/en/workshop.php
+++ b/mod/workshop/lang/en/workshop.php
@@ -53,6 +53,7 @@ $string['assessmentenddatetime'] = 'Assessment deadline: {$a->daydatetime} ({$a-
 $string['assessmentform'] = 'Assessment form';
 $string['assessmentofsubmission'] = '<a href="{$a->assessmenturl}">Assessment</a> of <a href="{$a->submissionurl}">{$a->submissiontitle}</a>';
 $string['assessmentreference'] = 'Reference assessment';
+$string['assessmentreferenceconflict'] = 'It is not possible to assess an example submission for which you provided a reference assessment.';
 $string['assessmentreferenceneeded'] = 'You have to assess this example submission to provide a reference assessment. Click \'Continue\' button to assess the submission.';
 $string['assessmentsettings'] = 'Assessment settings';
 $string['assessmentstart'] = 'Open for assessment from';