diff --git a/duplicate_keys_test.go b/duplicate_keys_test.go
new file mode 100644
index 00000000..b1d2ac61
--- /dev/null
+++ b/duplicate_keys_test.go
@@ -0,0 +1,90 @@
+package yaml_test
+
+import (
+	"bytes"
+
+	yaml "github.com/oasdiff/yaml3"
+	. "gopkg.in/check.v1"
+)
+
+type dupInfo struct {
+	Title   string `yaml:"title"`
+	Version string `yaml:"version"`
+}
+
+type dupDoc struct {
+	Info dupInfo `yaml:"info"`
+}
+
+const dupSrc = "info:\n  title: a\n  title: b\n  version: \"1\"\n"
+
+func decodeDup(c *C, allow bool) (dupDoc, error) {
+	dec := yaml.NewDecoder(bytes.NewBufferString(dupSrc))
+	dec.AllowDuplicateKeys(allow)
+	var d dupDoc
+	return d, dec.Decode(&d)
+}
+
+// The default is unchanged: a repeated key is an error, which is YAML's rule.
+func (s *S) TestDuplicateKeys_RejectedByDefault(c *C) {
+	dec := yaml.NewDecoder(bytes.NewBufferString(dupSrc))
+	var d dupDoc
+	err := dec.Decode(&d)
+	c.Assert(err, NotNil)
+	c.Assert(err.Error(), Matches, `(?s).*already defined.*`)
+}
+
+func (s *S) TestDuplicateKeys_RejectedWhenDisallowed(c *C) {
+	_, err := decodeDup(c, false)
+	c.Assert(err, NotNil)
+}
+
+// Enabled, the last occurrence wins -- the same answer encoding/json gives for
+// a repeated JSON object name. Asserting the surviving value, not merely the
+// absence of an error: "does not fail" would also be satisfied by dropping the
+// key or keeping the first, and neither would match encoding/json.
+func (s *S) TestDuplicateKeys_LastOneWinsWhenAllowed(c *C) {
+	d, err := decodeDup(c, true)
+	c.Assert(err, IsNil)
+	c.Assert(d.Info.Title, Equals, "b")
+	// Sibling keys are untouched by the override.
+	c.Assert(d.Info.Version, Equals, "1")
+}
+
+// The flag is per-decoder, so enabling it for one document must not leak into
+// another built from the same package.
+func (s *S) TestDuplicateKeys_FlagDoesNotLeak(c *C) {
+	_, err := decodeDup(c, true)
+	c.Assert(err, IsNil)
+
+	dec := yaml.NewDecoder(bytes.NewBufferString(dupSrc))
+	var d dupDoc
+	c.Assert(dec.Decode(&d), NotNil)
+}
+
+// A duplicate inside a nested mapping is caught too, not just at the top level.
+func (s *S) TestDuplicateKeys_Nested(c *C) {
+	const src = "a:\n  b:\n    c: 1\n    c: 2\n"
+	var out any
+
+	dec := yaml.NewDecoder(bytes.NewBufferString(src))
+	c.Assert(dec.Decode(&out), NotNil)
+
+	dec = yaml.NewDecoder(bytes.NewBufferString(src))
+	dec.AllowDuplicateKeys(true)
+	out = nil
+	c.Assert(dec.Decode(&out), IsNil)
+	a := out.(map[string]any)["a"].(map[string]any)
+	c.Assert(a["b"].(map[string]any)["c"], Equals, 2)
+}
+
+// Decoding into a generic map, rather than a struct, takes a different path in
+// the decoder (mapping vs mappingStruct), so it needs its own coverage.
+func (s *S) TestDuplicateKeys_IntoGenericMap(c *C) {
+	dec := yaml.NewDecoder(bytes.NewBufferString(dupSrc))
+	dec.AllowDuplicateKeys(true)
+	var out any
+	c.Assert(dec.Decode(&out), IsNil)
+	info := out.(map[string]any)["info"].(map[string]any)
+	c.Assert(info["title"], Equals, "b")
+}
diff --git a/yaml.go b/yaml.go
index f8095e30..fbe71267 100644
--- a/yaml.go
+++ b/yaml.go
@@ -89,11 +89,12 @@ func Unmarshal(in []byte, out interface{}) (err error) {
 
 // A Decoder reads and decodes YAML values from an input stream.
 type Decoder struct {
-	parser            *parser
-	knownFields       bool
-	origin            bool
-	file              string
-	disableTimestamps bool
+	parser             *parser
+	knownFields        bool
+	origin             bool
+	file               string
+	disableTimestamps  bool
+	allowDuplicateKeys bool
 }
 
 // NewDecoder returns a new decoder that reads from r.
@@ -106,6 +107,21 @@ func NewDecoder(r io.Reader) *Decoder {
 	}
 }
 
+// AllowDuplicateKeys controls what happens when a mapping repeats a key.
+//
+// By default a repeated key is an error, which is YAML's rule and this
+// package's long-standing behaviour. Enabling this makes the last occurrence
+// win instead, which is what encoding/json does with a repeated JSON object
+// name (RFC 8259 says names SHOULD be unique, so the handling is left to the
+// implementation).
+//
+// It exists for callers that decode both YAML and JSON into the same types and
+// need one answer for both, rather than a document loading or failing
+// depending on which format it happens to be written in.
+func (dec *Decoder) AllowDuplicateKeys(enable bool) {
+	dec.allowDuplicateKeys = enable
+}
+
 // KnownFields ensures that the keys in decoded mappings to
 // exist as fields in the struct being decoded into.
 func (dec *Decoder) KnownFields(enable bool) {
@@ -143,6 +159,7 @@ func (dec *Decoder) DisableTimestamps(disable bool) {
 func (dec *Decoder) Decode(v interface{}) (err error) {
 	d := newDecoder()
 	d.knownFields = dec.knownFields
+	d.uniqueKeys = !dec.allowDuplicateKeys
 	d.origin = dec.origin
 	d.file = dec.file
 	d.disableTimestamps = dec.disableTimestamps