diff --git a/Lib/multiprocessing/popen_spawn_posix.py b/Lib/multiprocessing/popen_spawn_posix.py
index 24b8634523e5f2..7c0377035baa6a 100644
--- a/Lib/multiprocessing/popen_spawn_posix.py
+++ b/Lib/multiprocessing/popen_spawn_posix.py
@@ -32,7 +32,9 @@ def __init__(self, process_obj):
         super().__init__(process_obj)
 
     def duplicate_for_child(self, fd):
-        self._fds.append(fd)
+        # duplicates in self._fds would later lead to a ValueError
+        if fd not in self._fds:
+            self._fds.append(fd)
         return fd
 
     def _launch(self, process_obj):
diff --git a/Misc/NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst b/Misc/NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst
new file mode 100644
index 00000000000000..87ec85faa69209
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-02-06-06-43-52.bpo-43142.Gt_6Yb.rst
@@ -0,0 +1 @@
+Do not add duplicate FDs to list in duplicate_for_child() in order to avoid ValueError: bad value(s) in fds_to_keep
\ No newline at end of file