diff --git a/BUGS.md b/BUGS.md
index 5b52e79..8b4c93e 100644
--- a/BUGS.md
+++ b/BUGS.md
@@ -31,6 +31,7 @@
 
 # FIXED
 
+- mesma ref no mesmo ficheiro não é detectado.
 - enter nas respostas mostra json
 - apos clicar no botao responder, inactivar o input (importante quando o tempo de correcção é grande)
 - double click submits twice.
diff --git a/aprendizations/learnapp.py b/aprendizations/learnapp.py
index 9d410d3..bbb3083 100644
--- a/aprendizations/learnapp.py
+++ b/aprendizations/learnapp.py
@@ -90,7 +90,6 @@ class LearnApp(object):
         for c, d in self.courses.items():
             for goal in d['goals']:
                 if goal not in self.deps.nodes():
-                    # logger.error(f'Goal "{goal}" of "{c}"" not in the graph')
                     raise LearnException(f'Goal "{goal}" from course "{c}" '
                                          ' does not exist')
 
@@ -409,14 +408,24 @@ class LearnApp(object):
             fullpath: str = path.join(topicpath, t['file'])
 
             logger.debug(f'  Loading {fullpath}')
-            questions: List[QDict] = load_yaml(fullpath, default=[])
+            # questions: List[QDict] = load_yaml(fullpath, default=[])
+            try:
+                questions: List[QDict] = load_yaml(fullpath)
+            except Exception:
+                raise LearnException(f'Failed to load "{fullpath}"')
 
             # update refs to include topic as prefix.
             # refs are required to be unique only within the file.
             # undefined are set to topic:n, where n is the question number
             # within the file
+            localrefs = set()  # refs in current file
             for i, q in enumerate(questions):
                 qref = q.get('ref', str(i))  # ref or number
+                if qref in localrefs:
+                    msg = f'Duplicate ref "{qref}" in "{topicpath}"'
+                    raise LearnException(msg)
+                localrefs.add(qref)
+
                 q['ref'] = f'{tref}:{qref}'
                 q['path'] = topicpath
                 q.setdefault('append_wrong', t['append_wrong'])
diff --git a/aprendizations/questions.py b/aprendizations/questions.py
index 93d639f..9a266ef 100644
--- a/aprendizations/questions.py
+++ b/aprendizations/questions.py
@@ -74,7 +74,14 @@ class QuestionRadio(Question):
     def __init__(self, q: QDict) -> None:
         super().__init__(q)
 
-        n = len(self['options'])
+        try:
+            n = len(self['options'])
+        except KeyError:
+            msg = f'Missing `options` in radio question. See {self["path"]}'
+            raise QuestionException(msg)
+        except TypeError:
+            msg = f'`options` must be a list. See {self["path"]}'
+            raise QuestionException(msg)
 
         self.set_defaults(QDict({
             'text': '',
@@ -185,7 +192,14 @@ class QuestionCheckbox(Question):
     def __init__(self, q: QDict) -> None:
         super().__init__(q)
 
-        n = len(self['options'])
+        try:
+            n = len(self['options'])
+        except KeyError:
+            msg = f'Missing `options` in radio question. See {self["path"]}'
+            raise QuestionException(msg)
+        except TypeError:
+            msg = f'`options` must be a list. See {self["path"]}'
+            raise QuestionException(msg)
 
         # set defaults if missing
         self.set_defaults(QDict({
diff --git a/aprendizations/tools.py b/aprendizations/tools.py
index f592545..0ec43f3 100644
--- a/aprendizations/tools.py
+++ b/aprendizations/tools.py
@@ -111,14 +111,13 @@ class HighlightRenderer(mistune.Renderer):
 
     def table(self, header, body):
         return ('<table class="table table-sm"><thead class="thead-light">'
-               f'{header}</thead><tbody>{body}</tbody></table>')
+                f'{header}</thead><tbody>{body}</tbody></table>')
 
     def image(self, src, title, alt):
         alt = mistune.escape(alt, quote=True)
         title = mistune.escape(title or '', quote=True)
         return (f'<img src="/file/{src}" alt="{alt}" title="{title}"'
-                 'class="img-fluid">')
-                # class="img-fluid mx-auto d-block"
+                f'class="img-fluid">')  # class="img-fluid mx-auto d-block"
 
     # Pass math through unaltered - mathjax does the rendering in the browser
     def block_math(self, text):
@@ -150,25 +149,22 @@ def load_yaml(filename: str, default: Any = None) -> Any:
     filename = path.expanduser(filename)
     try:
         f = open(filename, 'r', encoding='utf-8')
-    except FileNotFoundError:
-        logger.error(f'Cannot open "{filename}": not found')
-    except PermissionError:
-        logger.error(f'Cannot open "{filename}": no permission')
-    except OSError:
-        logger.error(f'Cannot open file "{filename}"')
-    else:
-        with f:
-            try:
-                default = yaml.safe_load(f)
-            except yaml.YAMLError as e:
-                if hasattr(e, 'problem_mark'):
-                    mark = e.problem_mark
-                    logger.error(f'File "{filename}" near line {mark.line+1}, '
-                                 f'column {mark.column+1}')
-                else:
-                    logger.error(f'File "{filename}"')
-    finally:
-        return default
+    except Exception as e:
+        logger.error(e)
+        if default is not None:
+            return default
+        else:
+            raise
+
+    with f:
+        try:
+            return yaml.safe_load(f)
+        except yaml.YAMLError as e:
+            logger.error(str(e).replace('\n', ' '))
+            if default is not None:
+                return default
+            else:
+                raise
 
 
 # ---------------------------------------------------------------------------
--
libgit2 0.21.2