err test for struct with duplicate member

test/err/struct-duplicate-member.c

@@ -0,0 +1,17 @@
+/* Ensure that a duplicate member in a struct produces an error.
+** https://github.com/cc65/cc65/issues/2015
+*/
+
+struct bads {
+    int a;
+    int a; /* this is an error */
+};
+
+union badu {
+    int a, a; /* also an error */
+};
+
+int main(void)
+{
+    return 0;
+}