diff --git a/lang/interpret_test.go b/lang/interpret_test.go
index a7fbb589..d229d31b 100644
--- a/lang/interpret_test.go
+++ b/lang/interpret_test.go
@@ -976,6 +976,8 @@ func TestAstFunc1(t *testing.T) {
 func TestAstFunc2(t *testing.T) {
 	const magicError = "# err: "
 	const magicError1 = "err1: "
+	const magicError9 = "err9: " // TODO: rename
+	// TODO: move them all down by one
 	const magicError2 = "err2: "
 	const magicError3 = "err3: "
 	const magicError4 = "err4: "
@@ -1010,6 +1012,8 @@ func TestAstFunc2(t *testing.T) {
 
 	type errs struct {
 		fail1 bool
+		fail9 bool // TODO: rename
+		// TODO: move them all down by one
 		fail2 bool
 		fail3 bool
 		fail4 bool
@@ -1068,6 +1072,8 @@ func TestAstFunc2(t *testing.T) {
 		// if the graph file has a magic error string, it's a failure
 		errStr := ""
 		fail1 := false
+		fail9 := false // TODO: rename
+		// TODO: move them all down by one
 		fail2 := false
 		fail3 := false
 		fail4 := false
@@ -1082,6 +1088,12 @@ func TestAstFunc2(t *testing.T) {
 				str = errStr
 				fail1 = true
 			}
+			if strings.HasPrefix(str, magicError9) { // TODO: rename
+				errStr = strings.TrimPrefix(str, magicError9)
+				str = errStr
+				fail9 = true
+			}
+			// TODO: move them all down by one
 			if strings.HasPrefix(str, magicError2) {
 				errStr = strings.TrimPrefix(str, magicError2)
 				str = errStr
@@ -1117,6 +1129,8 @@ func TestAstFunc2(t *testing.T) {
 			expstr: str,
 			errs: errs{
 				fail1: fail1,
+				fail9: fail9, // TODO: rename
+				// TODO: move them all down by one
 				fail2: fail2,
 				fail3: fail3,
 				fail4: fail4,
@@ -1156,6 +1170,8 @@ func TestAstFunc2(t *testing.T) {
 			name, path, fail, expstr, errs := tc.name, tc.path, tc.fail, strings.Trim(tc.expstr, "\n"), tc.errs
 			src := dir + path // location of the test
 			fail1 := errs.fail1
+			fail9 := errs.fail9 // TODO: rename
+			// TODO: move them all down by one
 			fail2 := errs.fail2
 			fail3 := errs.fail3
 			fail4 := errs.fail4
@@ -1282,11 +1298,28 @@ func TestAstFunc2(t *testing.T) {
 				},
 			}
 			// some of this might happen *after* interpolate in SetScope or Unify...
-			if err := ast.Init(data); err != nil {
+			err = ast.Init(data)
+			if (!fail || !fail9) && err != nil {
 				t.Errorf("test #%d: FAIL", index)
 				t.Errorf("test #%d: could not init and validate AST: %+v", index, err)
 				return
 			}
+			if fail9 && err != nil {
+				// TODO: %+v instead?
+				s := fmt.Sprintf("%s", err) // convert to string
+				if s != expstr {
+					t.Errorf("test #%d: FAIL", index)
+					t.Errorf("test #%d: expected different error", index)
+					t.Logf("test #%d: err: %s", index, s)
+					t.Logf("test #%d: exp: %s", index, expstr)
+				}
+				return // fail happened during lex parse, don't run init/interpolate!
+			}
+			if fail9 && err == nil {
+				t.Errorf("test #%d: FAIL", index)
+				t.Errorf("test #%d: Init passed, expected fail", index)
+				return
+			}
 
 			iast, err := ast.Interpolate()
 			if err != nil {
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate0.output b/lang/interpret_test/TestAstFunc2/struct-duplicate0.output
new file mode 100644
index 00000000..5d8fb049
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate0.output
@@ -0,0 +1 @@
+# err: err1: parser: `syntax error: unexpected $end, expecting EQUALS` @1:6
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate0/main.mcl b/lang/interpret_test/TestAstFunc2/struct-duplicate0/main.mcl
new file mode 100644
index 00000000..71281ae3
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate0/main.mcl
@@ -0,0 +1 @@
+$d []struct{x int;x int}
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate1.output b/lang/interpret_test/TestAstFunc2/struct-duplicate1.output
new file mode 100644
index 00000000..45eb0d8b
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate1.output
@@ -0,0 +1 @@
+# err: err1: can't set return type in parser: `can't set return type in parser: duplicate struct field of `x int`` @1:24
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate1/main.mcl b/lang/interpret_test/TestAstFunc2/struct-duplicate1/main.mcl
new file mode 100644
index 00000000..3c697e79
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate1/main.mcl
@@ -0,0 +1 @@
+$st struct{x int; x int} = struct{x => 0,}
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate2.output b/lang/interpret_test/TestAstFunc2/struct-duplicate2.output
new file mode 100644
index 00000000..45eb0d8b
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate2.output
@@ -0,0 +1 @@
+# err: err1: can't set return type in parser: `can't set return type in parser: duplicate struct field of `x int`` @1:24
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate2/main.mcl b/lang/interpret_test/TestAstFunc2/struct-duplicate2/main.mcl
new file mode 100644
index 00000000..49c11139
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate2/main.mcl
@@ -0,0 +1 @@
+$st struct{x int; x int} = struct{x => 0, x => 0,}
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate3.output b/lang/interpret_test/TestAstFunc2/struct-duplicate3.output
new file mode 100644
index 00000000..f2ad69a3
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate3.output
@@ -0,0 +1 @@
+# err: err9: duplicate struct field name of: `x`
diff --git a/lang/interpret_test/TestAstFunc2/struct-duplicate3/main.mcl b/lang/interpret_test/TestAstFunc2/struct-duplicate3/main.mcl
new file mode 100644
index 00000000..6931f4c9
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/struct-duplicate3/main.mcl
@@ -0,0 +1,2 @@
+$st struct{x str} = struct{x => "hello", x => "world",}
+test structlookup($st, "x") {}
diff --git a/lang/parser.y b/lang/parser.y
index 97cf44dd..569a1d1f 100644
--- a/lang/parser.y
+++ b/lang/parser.y
@@ -48,8 +48,6 @@ func init() {
 	int     int64 // this is the .int as seen in lexer.nex
 	float   float64
 
-	strSlice []string
-
 	typ   *types.Type
 
 	stmts []interfaces.Stmt
@@ -1177,7 +1175,23 @@ type:
 	// struct: struct{} or struct{a bool} or struct{a bool; bb int}
 	{
 		posLast(yylex, yyDollar) // our pos
-		$$.typ = types.NewType(fmt.Sprintf("%s{%s}", $1.str, strings.Join($3.strSlice, "; ")))
+
+		names := make(map[string]struct{})
+		strs := []string{}
+		for _, arg := range $3.args {
+			s := fmt.Sprintf("%s %s", arg.Name, arg.Type.String())
+			if _, exists := names[s]; exists {
+				// duplicate field name used
+				err := fmt.Errorf("duplicate struct field of `%s`", s)
+				// this will ultimately cause a parser error to occur...
+				yylex.Error(fmt.Sprintf("%s: %+v", ErrParseSetType, err))
+				break // we must skip, because code continues!
+			}
+			names[s] = struct{}{}
+			strs = append(strs, s)
+		}
+
+		$$.typ = types.NewType(fmt.Sprintf("%s{%s}", $1.str, strings.Join(strs, "; ")))
 	}
 |	FUNC_IDENTIFIER OPEN_PAREN type_func_args CLOSE_PAREN type
 	// XXX: should we allow named args in the type signature?
@@ -1228,24 +1242,27 @@ type_struct_fields:
 	/* end of list */
 	{
 		posLast(yylex, yyDollar) // our pos
-		$$.strSlice = []string{}
+		$$.args = []*Arg{}
 	}
 |	type_struct_fields SEMICOLON type_struct_field
 	{
 		posLast(yylex, yyDollar) // our pos
-		$$.strSlice = append($1.strSlice, $3.str)
+		$$.args = append($1.args, $3.arg)
 	}
 |	type_struct_field
 	{
 		posLast(yylex, yyDollar) // our pos
-		$$.strSlice = []string{$1.str}
+		$$.args = append([]*Arg{}, $1.arg)
 	}
 ;
 type_struct_field:
 	IDENTIFIER type
 	{
 		posLast(yylex, yyDollar) // our pos
-		$$.str = fmt.Sprintf("%s %s", $1.str, $2.typ.String())
+		$$.arg = &Arg{ // re-use the Arg struct
+			Name: $1.str,
+			Type: $2.typ,
+		}
 	}
 ;
 type_func_args:
diff --git a/lang/structs.go b/lang/structs.go
index 62efb553..b80d11c5 100644
--- a/lang/structs.go
+++ b/lang/structs.go
@@ -5730,6 +5730,7 @@ func (obj *ExprMap) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *ExprMap) Init(data *interfaces.Data) error {
+	// XXX: Can we check that there aren't any duplicate keys? Can we Cmp?
 	for _, x := range obj.KVs {
 		if err := x.Key.Init(data); err != nil {
 			return err
@@ -6252,7 +6253,14 @@ func (obj *ExprStruct) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *ExprStruct) Init(data *interfaces.Data) error {
+	fields := make(map[string]struct{})
 	for _, x := range obj.Fields {
+		// Validate field names and ensure no duplicates!
+		if _, exists := fields[x.Name]; exists {
+			return fmt.Errorf("duplicate struct field name of: `%s`", x.Name)
+		}
+		fields[x.Name] = struct{}{}
+
 		if err := x.Value.Init(data); err != nil {
 			return err
 		}