diff --git a/dotfiles/lib/python/merge_zsh_history.py b/dotfiles/lib/python/merge_zsh_history.py
index c7b5def4..e92f57aa 100644
--- a/dotfiles/lib/python/merge_zsh_history.py
+++ b/dotfiles/lib/python/merge_zsh_history.py
@@ -2,9 +2,24 @@ import re
 
 history_entry_re = re.compile(": ([0-9]*):[0-9]*;.*")
 
+
+class dedup(set):
+
+    def contains_add(self, elem):
+        if elem in self:
+            return True
+        else:
+            self.add(elem)
+            return False
+
+
 def merge_history(filenames):
+    entries = dedup()
     pairs = [entry for filename in filenames for entry in entry_timestamp_pairs(filename)]
-    return "".join([p[0] for p in sorted(pairs, key=lambda pair: pair[1])])
+    return "".join([
+        p[0] for p in sorted(pairs, key=lambda pair: pair[1])
+        if not entries.contains_add(p[0])
+    ])
 
 
 def entry_timestamp_pairs(filename):