diff --git a/dupkeys_test.go b/dupkeys_test.go
new file mode 100644
index 0000000..14f9eb1
--- /dev/null
+++ b/dupkeys_test.go
@@ -0,0 +1,56 @@
+package yaml_test
+
+import (
+	"testing"
+
+	"github.com/oasdiff/yaml"
+)
+
+type dupSpec struct {
+	Info struct {
+		Title   string `json:"title"`
+		Version string `json:"version"`
+	} `json:"info"`
+}
+
+const dupSrc = "info:\n  title: a\n  title: b\n  version: \"1\"\n"
+
+func TestDecodeOpts_DuplicateKeys(t *testing.T) {
+	var strict dupSpec
+	if _, err := yaml.Unmarshal([]byte(dupSrc), &strict, yaml.DecodeOpts{}); err == nil {
+		t.Fatal("default should reject a repeated key")
+	}
+
+	var lenient dupSpec
+	if _, err := yaml.Unmarshal([]byte(dupSrc), &lenient,
+		yaml.DecodeOpts{AllowDuplicateKeys: true}); err != nil {
+		t.Fatalf("AllowDuplicateKeys should accept: %v", err)
+	}
+	// The last occurrence wins, as encoding/json does. Asserting the value,
+	// not just the absence of an error.
+	if lenient.Info.Title != "b" {
+		t.Fatalf("title = %q, want %q", lenient.Info.Title, "b")
+	}
+	if lenient.Info.Version != "1" {
+		t.Fatalf("sibling key clobbered: version = %q", lenient.Info.Version)
+	}
+}
+
+// The option must survive the JSON round trip this package performs, and must
+// compose with origin tracking rather than disabling it.
+func TestDecodeOpts_DuplicateKeysWithOrigin(t *testing.T) {
+	var v dupSpec
+	tree, err := yaml.Unmarshal([]byte(dupSrc), &v, yaml.DecodeOpts{
+		AllowDuplicateKeys: true,
+		Origin:             yaml.OriginOpt{Enabled: true, File: "spec.yaml"},
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if v.Info.Title != "b" {
+		t.Fatalf("title = %q, want %q", v.Info.Title, "b")
+	}
+	if tree == nil {
+		t.Fatal("origin tree should still be produced")
+	}
+}
diff --git a/go.mod b/go.mod
index b094f80..8eb2f7b 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,4 @@ module github.com/oasdiff/yaml
 
 go 1.25
 
-require github.com/oasdiff/yaml3 v0.0.14
+require github.com/oasdiff/yaml3 v0.0.15-0.20260802195239-79d189934084
diff --git a/go.sum b/go.sum
index 9e6d2c6..c5a4812 100644
--- a/go.sum
+++ b/go.sum
@@ -1,4 +1,4 @@
-github.com/oasdiff/yaml3 v0.0.14 h1:aLJee3hxBK2H5wdXd9iPcIXb93Nty1Ge0pT171eHtkw=
-github.com/oasdiff/yaml3 v0.0.14/go.mod h1:csto2xfDjYccdUn/yw/bPjj/cYTdp6HtFA0J4TWG+gg=
+github.com/oasdiff/yaml3 v0.0.15-0.20260802195239-79d189934084 h1:0FFmaKhYkUtCiwh81RlE2WAmd8ga69YnnTOzJXHIKTE=
+github.com/oasdiff/yaml3 v0.0.15-0.20260802195239-79d189934084/go.mod h1:csto2xfDjYccdUn/yw/bPjj/cYTdp6HtFA0J4TWG+gg=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
diff --git a/yaml.go b/yaml.go
index 3ec5366..a767c8f 100644
--- a/yaml.go
+++ b/yaml.go
@@ -59,6 +59,13 @@ type DecodeOpts struct {
 	// real-world specs that use date-shaped strings as keys. Explicit
 	// "!!timestamp" tags in the source still resolve to time.Time.
 	DisableTimestamps bool
+	// AllowDuplicateKeys makes a repeated mapping key take its last value
+	// instead of being an error. That is what encoding/json does with a
+	// repeated JSON object name, so it is the setting to use when the same
+	// types are decoded from both YAML and JSON and a document should not
+	// load or fail purely on account of its format. Default false: repeated
+	// keys are an error, which is YAML's rule.
+	AllowDuplicateKeys bool
 }
 
 // OriginTree holds __origin__ data extracted from a YAML-decoded map tree.
@@ -87,6 +94,7 @@ type OriginTree struct {
 func Unmarshal(y []byte, o interface{}, decode DecodeOpts, opts ...JSONOpt) (*OriginTree, error) {
 	dec := yaml.NewDecoder(bytes.NewReader(y))
 	dec.Origin(decode.Origin.Enabled, decode.Origin.File)
+	dec.AllowDuplicateKeys(decode.AllowDuplicateKeys)
 	if decode.DisableTimestamps {
 		dec.DisableTimestamps(true)
 	}