Fix double <system-out> entry in junit.xml

check it system-out already exists
if this is the case append text

10841.improvement.rst

@@ -0,0 +1 @@
+only create <system-out> in junit.xml once for each test.

AUTHORS

@@ -189,6 +189,7 @@ Joshua Bronson
 Jurko Gospodnetić
 Justice Ndou
 Justyna Janczyszyn
+Kai Kasprowsky
 Kale Kundert
 Kamran Ahmad
 Karl O. Pinc

src/_pytest/junitxml.py

@@ -186,9 +186,18 @@ class _NodeReporter:
         return "\n".join([header.center(80, "-"), content, ""])
 
     def _write_content(self, report: TestReport, content: str, jheader: str) -> None:
-        tag = ET.Element(jheader)
-        tag.text = bin_xml_escape(content)
-        self.append(tag)
+        exists = False
+        for elem in next(iter(self.xml.node_reporters.values())).nodes:
+            if jheader in elem.tag:
+                exists = True
+                tag = elem
+                tag.text = (
+                    tag.text + bin_xml_escape(content) if tag.text is not None else None
+                )
+        if not exists:
+            tag = ET.Element(jheader)
+            tag.text = bin_xml_escape(content)
+            self.append(tag)
 
     def append_pass(self, report: TestReport) -> None:
         self.add_stats("passed")