diff --git a/testdata/p521-dup.txt b/testdata/p521-dup.txt
new file mode 100644
index 0000000..35f4676
--- /dev/null
+++ b/testdata/p521-dup.txt
@@ -0,0 +1,19 @@
+P521 example from RFC 9500:
+
+-----BEGIN EC PRIVATE KEY-----
+MIHcAgEBBEIB2STcygqIf42Zdno32HTmN6Esy0d9bghmU1ZpTWi3ZV5QaWOP3ntF
+yFQBPcd6NbGGVbhMlmpgIg1A+R7Z9RRYAuqgBwYFK4EEACOhgYkDgYYABAHQ/XJX
+qEx0f1YldcBzhdvr8vUr6lgIPbgv3RUx2KrjzIdf8C/3+i2iYNjrYtbS9dZJJ44y
+FzagYoy7swMItuYY2wD2KtIExkYDWbyBiriWG/Dw/A7FquikKBc85W8A3psVfB5c
+gsZPVi/K3vxKTCj200LPPvYW/ILTO3KFySHyvzb92A==
+-----END EC PRIVATE KEY-----
+
+The same P521 example from RFC 9500:
+
+-----BEGIN EC PRIVATE KEY-----
+MIHcAgEBBEIB2STcygqIf42Zdno32HTmN6Esy0d9bghmU1ZpTWi3ZV5QaWOP3ntF
+yFQBPcd6NbGGVbhMlmpgIg1A+R7Z9RRYAuqgBwYFK4EEACOhgYkDgYYABAHQ/XJX
+qEx0f1YldcBzhdvr8vUr6lgIPbgv3RUx2KrjzIdf8C/3+i2iYNjrYtbS9dZJJ44y
+FzagYoy7swMItuYY2wD2KtIExkYDWbyBiriWG/Dw/A7FquikKBc85W8A3psVfB5c
+gsZPVi/K3vxKTCj200LPPvYW/ILTO3KFySHyvzb92A==
+-----END EC PRIVATE KEY-----