diff --git a/setup.py b/setup.py
index 52aaec9..afdcf6f 100644
--- a/setup.py
+++ b/setup.py
@@ -5,7 +5,7 @@
 
 setuptools.setup(
     name="simple_coverage",
-    version="0.3.0",
+    version="0.3.1",
     author="Nikolai Limbrunner",
     author_email="nikolai.limbrunner@gmail.com",
     description="Simple Coverage measurements for Python",
diff --git a/simple_coverage/output.py b/simple_coverage/output.py
index 00bbc60..e365951 100644
--- a/simple_coverage/output.py
+++ b/simple_coverage/output.py
@@ -30,8 +30,10 @@ def save_coverage_output(func_name, instructions_dict, called_instructions):
 
         # check if function already exists
         if func_name in data:
-            # append called instructions to existing function
-            data[func_name]["called_instructions"].extend(called_instructions)
+            # append new called instructions to existing function
+            for instruction in called_instructions: 
+                if instruction not in data[func_name]["called_instructions"]:
+                    data[func_name]["called_instructions"].append(instruction)
         else:
             # otherwise create new entry for new function
             data.update(