diff --git a/news/output-wavelength.rst b/news/output-wavelength.rst
new file mode 100644
index 0000000..7522d38
--- /dev/null
+++ b/news/output-wavelength.rst
@@ -0,0 +1,23 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* duplicated wavelength information in output files
+
+**Security:**
+
+* <news item>
diff --git a/src/diffpy/labpdfproc/tools.py b/src/diffpy/labpdfproc/tools.py
index a4301ed..f5f31da 100644
--- a/src/diffpy/labpdfproc/tools.py
+++ b/src/diffpy/labpdfproc/tools.py
@@ -6,7 +6,10 @@
 
 WAVELENGTHS = {"Mo": 0.71, "Ag": 0.59, "Cu": 1.54}
 known_sources = [key for key in WAVELENGTHS.keys()]
-METADATA_KEYS_TO_EXCLUDE = ["output_correction", "force_overwrite", "input", "input_paths"]
+
+# Exclude wavelength from metadata to prevent duplication,
+# as the dump function in diffpy.utils writes it explicitly.
+METADATA_KEYS_TO_EXCLUDE = ["output_correction", "force_overwrite", "input", "input_paths", "wavelength"]
 
 
 def set_output_directory(args):
diff --git a/tests/test_tools.py b/tests/test_tools.py
index 67773e4..afb7bee 100644
--- a/tests/test_tools.py
+++ b/tests/test_tools.py
@@ -336,7 +336,6 @@ def test_load_metadata(mocker, user_filesystem):
             "mud": 2.5,
             "input_directory": str(filepath),
             "anode_type": "Mo",
-            "wavelength": 0.71,
             "output_directory": str(Path.cwd().resolve()),
             "xtype": "tth",
             "method": "polynomial_interpolation",