cmd/doc: don't print a declaration twice

That can occur if we have -u set and there is an upper- and lower-case
name of the same spelling in a single declaration.

A rare corner case but easy to fix.

Fix by remembering what we've printed.

Fixes #21797.

Change-Id: Ie0b681ae8c277fa16e9635ba594c1dff272b8aeb
Reviewed-on: https://go-review.googlesource.com/78715
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/doc/doc_test.go

@@ -445,6 +445,19 @@ var tests = []test{
 			`CaseMatch`,
 		},
 	},
+
+	// No dups with -u. Issue 21797.
+	{
+		"case matching on, no dups",
+		[]string{"-u", p, `duplicate`},
+		[]string{
+			`Duplicate`,
+			`duplicate`,
+		},
+		[]string{
+			"\\)\n+const", // This will appear if the const decl appears twice.
+		},
+	},
 }
 
 func TestDoc(t *testing.T) {