+test_expect_equal_file EXPECTED OUTPUT
+
+test_begin_subtest "--output=messages --format=json --duplicate=1"
+notmuch search --output=messages --format=json --duplicate=1 '*' >OUTPUT
+# reuse same EXPECTED as above
+test_expect_equal_file EXPECTED OUTPUT
+
+test_begin_subtest "--output=messages --format=json --duplicate=2"
+notmuch search --output=messages --format=json --duplicate=2 '*' >OUTPUT
+cat <<EOF >EXPECTED
+["20091117232137.GA7669@griffis1.net"]
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+test_begin_subtest "--output=messages --format=json --duplicate=3"
+notmuch search --output=messages --format=json --duplicate=3 '*' >OUTPUT
+cat <<EOF >EXPECTED
+[]
+EOF
+test_expect_equal_file EXPECTED OUTPUT