3 files changed, 25 insertions, 1 deletions
diff --git a/src/cmd/doc/doc_test.go b/src/cmd/doc/doc_test.go
index 0df53a3630..ee7c430cbd 100644
--- a/src/cmd/doc/doc_test.go
+++ b/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) {
diff --git a/src/cmd/doc/pkg.go b/src/cmd/doc/pkg.go
index 5a14d6e7cf..99a00c5632 100644
--- a/src/cmd/doc/pkg.go
+++ b/src/cmd/doc/pkg.go
@@ -594,6 +594,11 @@ func (pkg *Package) symbolDoc(symbol string) bool {
 	// Constants and variables behave the same.
 	values := pkg.findValues(symbol, pkg.doc.Consts)
 	values = append(values, pkg.findValues(symbol, pkg.doc.Vars)...)
+	// A declaration like
+	//	const ( c = 1; C = 2 )
+	// could be printed twice if the -u flag is set, as it matches twice.
+	// So we remember which declarations we've printed to avoid duplication.
+	printed := make(map[*ast.GenDecl]bool)
 	for _, value := range values {
 		// Print each spec only if there is at least one exported symbol in it.
 		// (See issue 11008.)
@@ -628,7 +633,7 @@ func (pkg *Package) symbolDoc(symbol string) bool {
 				}
 			}
 		}
-		if len(specs) == 0 {
+		if len(specs) == 0 || printed[value.Decl] {
 			continue
 		}
 		value.Decl.Specs = specs
@@ -636,6 +641,7 @@ func (pkg *Package) symbolDoc(symbol string) bool {
 			pkg.packageClause(true)
 		}
 		pkg.emit(value.Doc, value.Decl)
+		printed[value.Decl] = true
 		found = true
 	}
 	// Types.
diff --git a/src/cmd/doc/testdata/pkg.go b/src/cmd/doc/testdata/pkg.go
index 99755b11c5..d0995bbf7d 100644
--- a/src/cmd/doc/testdata/pkg.go
+++ b/src/cmd/doc/testdata/pkg.go
@@ -193,3 +193,8 @@ var LongLine = newLongLine(
 type T2 int
 
 type T1 = T2
+
+const (
+	Duplicate = iota
+	duplicate
+)