cgen: fix duplicate optional generation for struct field

vlib/v/gen/c/cgen.v

@@ -5297,14 +5297,17 @@ fn (mut g Gen) write_types(types []table.TypeSymbol) {
 						if field.typ.has_flag(.optional) {
 							// Dont use g.typ() here becuase it will register
 							// optional and we dont want that
+							styp, base := g.optional_type_name(field.typ)
+							if styp !in g.optionals {
 								last_text := g.type_definitions.after(start_pos).clone()
 								g.type_definitions.go_back_to(start_pos)
-							styp, base := g.optional_type_name(field.typ)
 								g.optionals << styp
 								g.typedefs2.writeln('typedef struct $styp $styp;')
-							g.type_definitions.writeln('${g.optional_type_text(styp, base)};')
+								g.type_definitions.writeln('${g.optional_type_text(styp,
+									base)};')
 								g.type_definitions.write(last_text)
 							}
+						}
 						type_name := g.typ(field.typ)
 						field_name := c_name(field.name)
 						g.type_definitions.writeln('\t$type_name $field_name;')

vlib/v/tests/option_test.v

@@ -372,3 +372,8 @@ fn test_optional_sum_type() {
 	}
 	assert false
 }
+
+struct MultiOptionalFieldTest {
+	a ?int
+	b ?int
+}