diff --git a/src/calibre/utils/formatter_functions.py b/src/calibre/utils/formatter_functions.py
index f72e26b19d..38fcbe8a1e 100644
--- a/src/calibre/utils/formatter_functions.py
+++ b/src/calibre/utils/formatter_functions.py
@@ -909,12 +909,10 @@ class BuiltinListUnion(BuiltinFormatterFunction):
         l2 = [l.strip() for l in list2.split(separator) if l.strip()]
         lcl1 = set([icu_lower(l) for l in l1])
 
-        res = []
-        for i in l1:
-            res.append(i)
+        res = set(l1)
         for i in l2:
             if icu_lower(i) not in lcl1:
-                res.append(i)
+                res.add(i)
         if separator == ',':
             return ', '.join(res)
         return separator.join(res)
@@ -930,7 +928,7 @@ class BuiltinListDifference(BuiltinFormatterFunction):
 
     def evaluate(self, formatter, kwargs, mi, locals, list1, list2, separator):
         l1 = [l.strip() for l in list1.split(separator) if l.strip()]
-        l2 = [icu_lower(l.strip()) for l in list2.split(separator) if l.strip()]
+        l2 = set([icu_lower(l.strip()) for l in list2.split(separator) if l.strip()])
 
         res = []
         for i in l1:
@@ -951,12 +949,12 @@ class BuiltinListIntersection(BuiltinFormatterFunction):
 
     def evaluate(self, formatter, kwargs, mi, locals, list1, list2, separator):
         l1 = [l.strip() for l in list1.split(separator) if l.strip()]
-        l2 = [icu_lower(l.strip()) for l in list2.split(separator) if l.strip()]
+        l2 = set([icu_lower(l.strip()) for l in list2.split(separator) if l.strip()])
 
-        res = []
+        res = set()
         for i in l1:
             if icu_lower(i) in l2:
-                res.append(i)
+                res.add(i)
         if separator == ',':
             return ', '.join(res)
         return separator.join(res)
@@ -1007,12 +1005,13 @@ class BuiltinListRe(BuiltinFormatterFunction):
 
     def evaluate(self, formatter, kwargs, mi, locals, src_list, separator, search_re, opt_replace):
         l = [l.strip() for l in src_list.split(separator) if l.strip()]
-        res = []
+        res = set()
         for item in l:
             if re.search(search_re, item, flags=re.I) is not None:
                 if opt_replace:
                     item = re.sub(search_re, opt_replace, item)
-                res.append(item)
+                for i in [l.strip() for l in item.split(',') if l.strip()]:
+                    res.add(i)
         if separator == ',':
             return ', '.join(res)
         return separator.join(res)