Fix for header repetition.

Not yet tested. See issue #342

src/sphinxcontrib/bibtex/domain.py

@@ -311,6 +311,7 @@ def __init__(self, env: "BuildEnvironment"):
         # parse bibliography header
         header = getattr(env.app.config, "bibtex_bibliography_header")
         if header:
+            self.data["bibliography_header"] = docutils.nodes.container()
             self.data["bibliography_header"] += parse_header(
                 header, "bibliography_header"
             )