diff --git a/internal/tracker/seen.go b/internal/tracker/seen.go
index af70241..8990dae 100644
--- a/internal/tracker/seen.go
+++ b/internal/tracker/seen.go
@@ -106,7 +106,6 @@ func (s *SeenTracker) create(parentIdx int, name []byte, kind keyKind, explicit
 // that have been seen in previous calls, and validates that types are consistent.
 func (s *SeenTracker) CheckExpression(node *ast.Node) error {
 	if s.entries == nil {
-		// s.entries = make([]entry, 0, 8)
 		// Skip ID = 0 to remove the confusion between nodes whose parent has
 		// id 0 and root nodes (parent id is 0 because it's the zero value).
 		s.nextID = 1
@@ -221,6 +220,9 @@ func (s *SeenTracker) checkKeyValue(node *ast.Node) error {
 			if s.entries[idx].kind != tableKind {
 				return fmt.Errorf("toml: expected %s to be a table, not a %s", string(k), s.entries[idx].kind)
 			}
+			if s.entries[idx].explicit {
+				return fmt.Errorf("toml: cannot redefine table %s that has already been explicitly defined", string(k))
+			}
 		} else {
 			idx = s.create(parentIdx, k, tableKind, false)
 		}
diff --git a/toml_testgen_test.go b/toml_testgen_test.go
index 3f3bcf5..61985ff 100644
--- a/toml_testgen_test.go
+++ b/toml_testgen_test.go
@@ -886,13 +886,11 @@ func TestTOMLTest_Invalid_Table_EqualsSign(t *testing.T) {
 }
 
 func TestTOMLTest_Invalid_Table_Injection1(t *testing.T) {
-	t.Skip("FIXME")
 	input := "[a.b.c]\n  z = 9\n[a]\n  b.c.t = \"Using dotted keys to add to [a.b.c] after explicitly defining it above is not allowed\"\n  \n# see https://github.com/toml-lang/toml/issues/846\n"
 	testgenInvalid(t, input)
 }
 
 func TestTOMLTest_Invalid_Table_Injection2(t *testing.T) {
-	t.Skip("FIXME")
 	input := "[a.b.c.d]\n  z = 9\n[a]\n  b.c.d.k.t = \"Using dotted keys to add to [a.b.c.d] after explicitly defining it above is not allowed\"\n  \n# see https://github.com/toml-lang/toml/issues/846\n"
 	testgenInvalid(t, input)
 }
diff --git a/unmarshaler.go b/unmarshaler.go
index dbe7cb0..19efaa4 100644
--- a/unmarshaler.go
+++ b/unmarshaler.go
@@ -516,6 +516,11 @@ func (d *decoder) handleKeyValues(v reflect.Value) (reflect.Value, error) {
 			break
 		}
 
+		err := d.seen.CheckExpression(expr)
+		if err != nil {
+			return reflect.Value{}, err
+		}
+
 		x, err := d.handleKeyValue(expr, v)
 		if err != nil {
 			return reflect.Value{}, err