diff --git a/anonymize_entries.py b/anonymize_entries.py
index 79f846a..af91628 100644
--- a/anonymize_entries.py
+++ b/anonymize_entries.py
@@ -145,16 +145,24 @@ def maybe_generate_aliases(
     if reuse_aliases:
         print("Reusing generated aliases")
         return False
-    else:
-        for row in csv_contents:
-            random = Random(
-                "; ".join([row[KnownColumns.EmailAddress], args.csv, args.files])
+
+    alias_to_email_address = {}
+
+    for row in csv_contents:
+        rnd = Random("; ".join([row[KnownColumns.EmailAddress], args.csv, args.files]))
+        random_alias = f"{rnd.choice(alias_parts[0])} {rnd.choice(alias_parts[0])}"
+        while (
+            random_alias in alias_to_email_address
+            and alias_to_email_address[random_alias] != row[KnownColumns.EmailAddress]
+        ):
+            print(
+                f"WARNING: rerolling alias for {row[KnownColumns.EmailAddress]} due to collision with {alias_to_email_address[random_alias]}"
             )
-            row[KnownColumns.GeneratedAlias] = (
-                f"{random.choice(alias_parts[0])} {random.choice(alias_parts[0])}"
-            )
-        print("Generated an alias for each entry")
-        return True
+            random_alias = f"{rnd.choice(alias_parts[0])} {rnd.choice(alias_parts[0])}"
+        row[KnownColumns.GeneratedAlias] = random_alias
+
+    print("Generated an alias for each entry")
+    return True
 
 
 def maybe_mark_resubmitted_entries(csv_contents: CsvContents) -> ChangedCsvContents: