diff --git a/hcl/parser/parser.go b/hcl/parser/parser.go
index 64c83bcf..985ecef0 100644
--- a/hcl/parser/parser.go
+++ b/hcl/parser/parser.go
@@ -76,6 +76,7 @@ func (p *Parser) objectList(obj bool) (*ast.ObjectList, error) {
 	defer un(trace(p, "ParseObjectList"))
 	node := &ast.ObjectList{}
 
+	seenKeys := map[string]struct{}{}
 	for {
 		if obj {
 			tok := p.scan()
@@ -83,11 +84,26 @@ func (p *Parser) objectList(obj bool) (*ast.ObjectList, error) {
 			if tok.Type == token.RBRACE {
 				break
 			}
+
 		}
 
 		n, err := p.objectItem()
+
 		if err == errEofToken {
 			break // we are finished
+		} else if err != nil {
+			return nil, err
+		}
+
+		if n.Assign.String() != "-" {
+			for _, key := range n.Keys {
+				_, ok := seenKeys[key.Token.Text]
+				if ok {
+					return nil, errors.New(fmt.Sprintf("The argument %q at %s was already set. Each argument can only be defined once", key.Token.Text, key.Token.Pos.String()))
+
+				}
+				seenKeys[key.Token.Text] = struct{}{}
+			}
 		}
 
 		// we don't return a nil node, because might want to use already
@@ -225,6 +241,7 @@ func (p *Parser) objectItem() (*ast.ObjectItem, error) {
 func (p *Parser) objectKey() ([]*ast.ObjectKey, error) {
 	keyCount := 0
 	keys := make([]*ast.ObjectKey, 0)
+	seenKeys := map[string]struct{}{}
 
 	for {
 		tok := p.scan()
@@ -268,6 +285,12 @@ func (p *Parser) objectKey() ([]*ast.ObjectKey, error) {
 			// object
 			return keys, err
 		case token.IDENT, token.STRING:
+			_, ok := seenKeys[p.tok.Text]
+			if ok {
+				return nil, errors.New(fmt.Sprintf("The argument %q at %s was already set. Each argument can only be defined once", p.tok.Text, p.tok.Pos.String()))
+			}
+			seenKeys[p.tok.Text] = struct{}{}
+
 			keyCount++
 			keys = append(keys, &ast.ObjectKey{Token: p.tok})
 		case token.ILLEGAL:
@@ -324,6 +347,8 @@ func (p *Parser) objectType() (*ast.ObjectType, error) {
 	// not a RBRACE, it's an syntax error and we just return it.
 	if err != nil && p.tok.Type != token.RBRACE {
 		return nil, err
+	} else if err != nil {
+		return nil, err
 	}
 
 	// No error, scan and expect the ending to be a brace
@@ -365,6 +390,7 @@ func (p *Parser) listType() (*ast.ListType, error) {
 		}
 		switch tok.Type {
 		case token.BOOL, token.NUMBER, token.FLOAT, token.STRING, token.HEREDOC:
+
 			node, err := p.literalType()
 			if err != nil {
 				return nil, err
diff --git a/hcl/parser/parser_test.go b/hcl/parser/parser_test.go
index 1188d09d..3e3d1f5c 100644
--- a/hcl/parser/parser_test.go
+++ b/hcl/parser/parser_test.go
@@ -134,6 +134,53 @@ func TestListOfMaps(t *testing.T) {
 	}
 }
 
+func TestDuplicateKeys_NotAllowed(t *testing.T) {
+	src := `key = "value1"
+			key = "value2"
+  `
+	p := newParser([]byte(src))
+
+	_, err := p.Parse()
+	if err == nil {
+		t.Fatalf("Expected error, got none!")
+	}
+
+	expected := "Each argument can only be defined once"
+	if !strings.Contains(err.Error(), expected) {
+		t.Fatalf("Expected err:\n  %s\nTo contain:\n  %s\n", err, expected)
+	}
+}
+
+func TestDuplicateKeys_NotAllowedInBlock(t *testing.T) {
+	src := `key = "value1"
+			structkey "structname" { name = "test" name = "test2"}
+  `
+	p := newParser([]byte(src))
+
+	_, err := p.Parse()
+	if err == nil {
+		t.Fatalf("Expected error, got none!")
+	}
+
+	expected := "Each argument can only be defined once"
+	if !strings.Contains(err.Error(), expected) {
+		t.Fatalf("Expected err:\n  %s\nTo contain:\n  %s\n", err, expected)
+	}
+}
+
+func TestDuplicateBlocks_allowed(t *testing.T) {
+	src := `structkey  { name = "test" }
+			structkey  { name = "test" }
+  `
+	p := newParser([]byte(src))
+
+	_, err := p.Parse()
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
+}
+
 func TestListOfMaps_requiresComma(t *testing.T) {
 	src := `foo = [
     {key = "bar"}
@@ -240,7 +287,7 @@ func TestListType_lineComment(t *testing.T) {
 			comment := l.comment[i]
 
 			if (lt.LineComment == nil) != (comment == "") {
-				t.Fatalf("bad: %s", lt.Token.Value())
+				t.Fatalf("bad: %v", lt)
 			}
 
 			if comment == "" {
diff --git a/hcl/parser/test-fixtures/types.hcl b/hcl/parser/test-fixtures/types.hcl
index cf2747ea..f403da1f 100644
--- a/hcl/parser/test-fixtures/types.hcl
+++ b/hcl/parser/test-fixtures/types.hcl
@@ -1,7 +1,7 @@
 foo = "bar"
 bar = 7
 baz = [1,2,3]
-foo = -12
-bar = 3.14159
-foo = true
-bar = false
+foo2 = -12
+bar2 = 3.14159
+foo3 = true
+bar3 = false