Don't allow tables to be redefined

parser.go

diff --git a/parser.go b/parser.go
index 54750c3..f91e80c 100644
--- a/parser.go
+++ b/parser.go
@@ -87,6 +87,9 @@
 	if key.typ != tokenKeyGroup {
 		panic(fmt.Sprintf("unexpected token %s, was expecting a key group", key))
 	}
+	if p.tree.Has(key.val) {
+		panic("duplicated tables")
+	}
 	p.tree.createSubTree(key.val)
 	p.assume(tokenRightBracket)
 	p.currentGroup = key.val

parser_test.go

diff --git a/parser_test.go b/parser_test.go
index 1f55faa..cda0322 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -196,6 +196,13 @@
 	})
 }
 
+func TestDuplicateGroups(t *testing.T) {
+	_, err := Load("[foo]\na=2\n[foo]b=3")
+	if err.Error() != "duplicated tables" {
+		t.Error("Bad error message:", err.Error())
+	}
+}
+
 func TestMissingFile(t *testing.T) {
 	_, err := LoadFile("foo.toml")
 	if err.Error() != "open foo.toml: no such file or directory" {

toml.go

diff --git a/toml.go b/toml.go
index 410bab6..bfd12b2 100644
--- a/toml.go
+++ b/toml.go
@@ -15,6 +15,18 @@
 // This is the result of the parsing of a TOML file.
 type TomlTree map[string]interface{}
 
+// Has returns a boolean indicating if the toplevel tree contains the given
+// key.
+func (t *TomlTree) Has(key string) bool {
+	mp := (map[string]interface{})(*t)
+	for k, _ := range mp {
+		if k == key {
+			return true
+		}
+	}
+	return false
+}
+
 // Keys returns the keys of the toplevel tree.
 // Warning: this is a costly operation.
 func (t *TomlTree) Keys() []string {