diff --git a/pkg/cmd/api/fields.go b/pkg/cmd/api/fields.go
index c9048a648..024eb12ca 100644
--- a/pkg/cmd/api/fields.go
+++ b/pkg/cmd/api/fields.go
@@ -99,6 +99,9 @@ func parseFields(opts *ApiOptions) (map[string]interface{}, error) {
 				}
 			}
 		} else {
+			if _, exists := destMap[subkey]; exists {
+				return fmt.Errorf("unexpected override existing field under %q", subkey)
+			}
 			destMap[subkey] = value
 		}
 		return nil
diff --git a/pkg/cmd/api/fields_test.go b/pkg/cmd/api/fields_test.go
index 49bda927d..73bc7463c 100644
--- a/pkg/cmd/api/fields_test.go
+++ b/pkg/cmd/api/fields_test.go
@@ -11,6 +11,7 @@ import (
 	"github.com/cli/cli/v2/internal/ghrepo"
 	"github.com/cli/cli/v2/pkg/iostreams"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func Test_parseFields(t *testing.T) {
@@ -135,6 +136,91 @@ func Test_parseFields_nested(t *testing.T) {
 	`), "\n"), string(jsonData))
 }
 
+func Test_parseFields_errors(t *testing.T) {
+	ios, stdin, _, _ := iostreams.Test()
+	fmt.Fprint(stdin, "pasted contents")
+
+	tests := []struct {
+		name     string
+		opts     *ApiOptions
+		expected string
+	}{
+		{
+			name: "cannot overwrite string to array",
+			opts: &ApiOptions{
+				IO: ios,
+				RawFields: []string{
+					"object[field]=A",
+					"object[field][]=this should be an error",
+				},
+			},
+			expected: `expected array type under "field", got string`,
+		},
+		{
+			name: "cannot overwrite string to object",
+			opts: &ApiOptions{
+				IO: ios,
+				RawFields: []string{
+					"object[field]=B",
+					"object[field][field2]=this should be an error",
+				},
+			},
+			expected: `expected map type under "field", got string`,
+		},
+		{
+			name: "cannot overwrite object to string",
+			opts: &ApiOptions{
+				IO: ios,
+				RawFields: []string{
+					"object[field][field2]=C",
+					"object[field]=this should be an error",
+				},
+			},
+			expected: `unexpected override existing field under "field"`,
+		},
+		{
+			name: "cannot overwrite object to array",
+			opts: &ApiOptions{
+				IO: ios,
+				RawFields: []string{
+					"object[field][field2]=D",
+					"object[field][]=this should be an error",
+				},
+			},
+			expected: `expected array type under "field", got map[string]interface {}`,
+		},
+		{
+			name: "cannot overwrite array to string",
+			opts: &ApiOptions{
+				IO: ios,
+				RawFields: []string{
+					"object[field][]=E",
+					"object[field]=this should be an error",
+				},
+			},
+			expected: `unexpected override existing field under "field"`,
+		},
+		{
+			name: "cannot overwrite array to object",
+			opts: &ApiOptions{
+				IO: ios,
+				RawFields: []string{
+					"object[field][]=F",
+					"object[field][field2]=this should be an error",
+				},
+			},
+			expected: `expected map type under "field", got []interface {}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := parseFields(tt.opts)
+			require.EqualError(t, err, tt.expected)
+		})
+	}
+}
+
 func Test_magicFieldValue(t *testing.T) {
 	f, err := os.CreateTemp(t.TempDir(), "gh-test")
 	if err != nil {