diff --git a/docxtpl/subdoc.py b/docxtpl/subdoc.py
index e8da093..f4ea439 100644
--- a/docxtpl/subdoc.py
+++ b/docxtpl/subdoc.py
@@ -7,6 +7,7 @@
 
 from docx import Document
 from docx.oxml import CT_SectPr
+from docx.oxml.ns import nsmap
 from docx.opc.constants import RELATIONSHIP_TYPE as RT
 from docxcompose.properties import CustomProperties
 from docxcompose.utils import xpath
@@ -16,6 +17,12 @@
 import re
 
 
+# docxcompose changes python-docx's process-wide ``cp`` prefix to the custom
+# properties namespace. Restore its standard meaning so python-docx updates
+# existing core properties instead of adding conflicting elements.
+nsmap["cp"] = "http://schemas.openxmlformats.org/package/2006/metadata/core-properties"
+
+
 class SubdocComposer(Composer):
     def attach_parts(self, doc, remove_property_fields=True):
         """Attach docx parts instead of appending the whole document
diff --git a/tests/core_properties.py b/tests/core_properties.py
new file mode 100644
index 0000000..0cd3a57
--- /dev/null
+++ b/tests/core_properties.py
@@ -0,0 +1,27 @@
+from zipfile import ZipFile
+
+from lxml import etree
+
+from docxtpl import DocxTemplate
+
+doctemplate = r"templates/doc_properties_tpl.docx"
+output = "output/core_properties.docx"
+
+tpl = DocxTemplate(doctemplate)
+tpl.render({"test": "HelloWorld"})
+tpl.docx.core_properties.keywords = "rendered by docxtpl"
+tpl.save(output)
+
+with ZipFile(output) as archive:
+    core_properties = etree.fromstring(archive.read("docProps/core.xml"))
+
+keywords = [
+    element
+    for element in core_properties
+    if etree.QName(element).localname == "keywords"
+]
+assert len(keywords) == 1
+assert etree.QName(keywords[0]).namespace == (
+    "http://schemas.openxmlformats.org/package/2006/metadata/core-properties"
+)
+assert keywords[0].text == "rendered by docxtpl"