diff --git a/internal/tracker/tracker.go b/internal/tracker/tracker.go
new file mode 100644
index 0000000..8359e2a
--- /dev/null
+++ b/internal/tracker/tracker.go
@@ -0,0 +1,98 @@
+package tracker
+
+import (
+	"fmt"
+
+	"github.com/pelletier/go-toml/v2/internal/ast"
+)
+
+type keyKind uint8
+
+const (
+	invalid keyKind = iota // also used for the root key
+	value
+	table
+	arrayTable
+)
+
+type key string
+
+type builder struct {
+	prefix [][]byte
+	local  [][]byte
+}
+
+func (b *builder) Reset(prefix [][]byte) {
+	b.prefix = prefix
+	b.local = b.local[:0]
+}
+
+// Computes the number of bytes required to store the full key.
+func (b *builder) size() int {
+	size := len(b.prefix) + len(b.local) - 1
+	for _, p := range b.prefix {
+		size += len(p)
+	}
+	for _, p := range b.local {
+		size += len(p)
+	}
+	return size
+}
+
+func (b *builder) copy(firstJoin bool, from [][]byte, to []byte) int {
+	offset := 0
+	for i, p := range from {
+		if i > 0 || firstJoin {
+			to[offset] = 0x1E
+			offset++
+		}
+		copy(to[offset:], p)
+		offset += len(p)
+	}
+	return offset
+}
+
+func (b *builder) MakeKey() key {
+	k := make([]byte, b.size())
+	b.copy(false, b.prefix, k)
+	b.copy(len(b.prefix) > 0, b.local, k)
+	return key(k)
+}
+
+func (b *builder) Append(k []byte) {
+	b.local = append(b.local, k)
+}
+
+// Tracks which keys have been seen with which TOML type to flag duplicates
+// and mismatches according to the spec.
+type Seen struct {
+	keys map[key]keyKind
+
+	// scoping from the previous CheckExpression call.
+	current [][]byte
+
+	// key builder
+	builder builder
+}
+
+// CheckExpression takes a top-level node and checks that it does not contain keys
+// that have been seen in previous calls, and validates that types are consistent.
+func (s *Seen) CheckExpression(node ast.Node) error {
+	s.builder.Reset(s.current)
+	switch node.Kind {
+	case ast.KeyValue:
+		return s.checkKeyValue(node)
+	case ast.Table:
+	case ast.ArrayTable:
+	default:
+		panic(fmt.Errorf("this should not be a top level node type: %s", node.Kind))
+	}
+	return nil
+}
+
+func (s *Seen) checkKeyValue(node ast.Node) error {
+	it := node.Key()
+	for it.Next() {
+		s.builder.Append(it.Node().Data)
+	}
+}
diff --git a/unmarshaler.go b/unmarshaler.go
index 31d12e6..59c6334 100644
--- a/unmarshaler.go
+++ b/unmarshaler.go
@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/pelletier/go-toml/v2/internal/ast"
+	"github.com/pelletier/go-toml/v2/internal/tracker"
 )
 
 func Unmarshal(data []byte, v interface{}) error {
@@ -19,6 +20,9 @@ func Unmarshal(data []byte, v interface{}) error {
 type decoder struct {
 	// Tracks position in Go arrays.
 	arrayIndexes map[reflect.Value]int
+
+	// Tracks keys that have been seen, with which type.
+	seen tracker.Seen
 }
 
 func (d *decoder) arrayIndex(append bool, v reflect.Value) int {
@@ -46,19 +50,25 @@ func (d *decoder) FromParser(p *parser, v interface{}) error {
 		return fmt.Errorf("target pointer must be non-nil")
 	}
 
-	var err error
 	var skipUntilTable bool
 	var root target = valueTarget(r.Elem())
 	current := root
 
 	for p.NextExpression() {
 		node := p.Expression()
+
+		if node.Kind == ast.KeyValue && skipUntilTable {
+			continue
+		}
+
+		err := d.seen.CheckExpression(node)
+		if err != nil {
+			return err
+		}
+
 		var found bool
 		switch node.Kind {
 		case ast.KeyValue:
-			if skipUntilTable {
-				continue
-			}
 			err = d.unmarshalKeyValue(current, node)
 			found = true
 		case ast.Table: