diff --git a/ext/typeexpr/get_type.go b/ext/typeexpr/get_type.go
index 64e3995a..8535933f 100644
--- a/ext/typeexpr/get_type.go
+++ b/ext/typeexpr/get_type.go
@@ -6,9 +6,10 @@ package typeexpr
 import (
 	"fmt"
 
-	"github.com/hashicorp/hcl/v2"
 	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/convert"
+
+	"github.com/hashicorp/hcl/v2"
 )
 
 const invalidTypeSummary = "Invalid type specification"
@@ -179,6 +180,18 @@ func getType(expr hcl.Expression, constraint, withDefaults bool) (cty.Type, *Def
 				})
 				continue
 			}
+
+			if _, exists := atys[attrName]; exists {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  invalidTypeSummary,
+					Detail:   "Object constructor map keys must be unique.",
+					Subject:  attrDef.Key.Range().Ptr(),
+					Context:  expr.Range().Ptr(),
+				})
+				continue
+			}
+
 			atyExpr := attrDef.Value
 
 			// the attribute type expression might be wrapped in the special
diff --git a/ext/typeexpr/get_type_test.go b/ext/typeexpr/get_type_test.go
index 7976bb87..d6f44844 100644
--- a/ext/typeexpr/get_type_test.go
+++ b/ext/typeexpr/get_type_test.go
@@ -10,10 +10,11 @@ import (
 	"github.com/hashicorp/hcl/v2/gohcl"
 
 	"github.com/google/go-cmp/cmp"
+	"github.com/zclconf/go-cty/cty"
+
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/hcl/v2/hclsyntax"
 	"github.com/hashicorp/hcl/v2/json"
-	"github.com/zclconf/go-cty/cty"
 )
 
 var (
@@ -700,6 +701,30 @@ func TestGetTypeDefaults(t *testing.T) {
 			nil,
 			`Optional attribute modifier expects at most two arguments: the attribute type, and a default value.`,
 		},
+
+		// Duplicate arguments.
+		{
+			`map(object({operations=optional(list(string), []),type=optional(string, "ABC"),type=optional(number)}))`,
+			&Defaults{
+				Type: cty.Map(cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+					"operations": cty.List(cty.String),
+					"type":       cty.String,
+				}, []string{"operations", "type"})),
+				Children: map[string]*Defaults{
+					"": {
+						Type: cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+							"operations": cty.List(cty.String),
+							"type":       cty.String,
+						}, []string{"operations", "type"}),
+						DefaultValues: map[string]cty.Value{
+							"operations": cty.ListValEmpty(cty.String),
+							"type":       cty.StringVal("ABC"),
+						},
+					},
+				},
+			},
+			"Object constructor map keys must be unique.",
+		},
 	}
 
 	for _, test := range tests {