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diff --git a/doc/textutils.texi b/doc/textutils.texi
index dbb8393ac3a57b0fa362458e82f8856e54882f31..032f2bdfbc4c67664c0b64a4d5f87349b7864ba1 100644 (file)
--- a/doc/textutils.texi
+++ b/doc/textutils.texi
@@ -2337,6 +2337,7 @@ Print only duplicate lines.
 Print all duplicate lines and only duplicate lines.
 This option is useful mainly in conjunction with other options e.g.,
 to ignore case or to compare only selected fields.
+This is a GNU extension.
 @c FIXME: give an example showing *how* it's useful
 
 @item -u