diff --git a/doc/conf.py b/doc/conf.py
index af76fe703bdc18dbf79f0726387c5fe947a994bb..a2a2ef0194b09435d7e208d0a33e2c16b438e197 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -99,7 +99,11 @@ def prepare():
     )
     os.environ.update(json.loads(out))
 
-prepare()
+# Only the top-level import has the "builtins" __name__. This prevents
+# re-running prepare() when conf.py is imported by the processes spawned by
+# sphinx
+if __name__ == 'builtins':
+    prepare()
 
 # -- General configuration ------------------------------------------------