I'm a Marriott associate (agm) , thought I'd chime in and clear up misinformation.
First and foremost, anyone who has an old copy of the form don't event bother trying to use it. Save yourself from embarrassment. They updated the forms in 2017 to include a new verification process at the time of check-in. Every form as a unique code in addition to the associates information.
Now to the OP. While the associate can not see where you have used it, if your second trip is after the first one you run the risk of them disabling your form before your next date of travel. Yes, the form indicates an expiration date, but the same portal that they created the form had the ability to disable it. And I personally go in and disable the forms the day after I know it was used to check in. For the very reason I do not want someone using the form and not letting me know. Their behavior could cost me my benefit, or worse my job. But that is also why you do not extend the discount to people that you cannot trust.
In fairness you are using a perk of their job. One which they were nice enough to extend to you. You shouldn't hide the fact that you are using the form elsewhere. You should be completely honest and open with them. Otherwise they may not react well if they do find out you used it, and now may not give you a form in the future.