Once upon a time, I was in a relatively new (less than a year) relationship, and it was coming up on our first Christmas together. It was many years ago, and I still found Love Actually extremely cute, instead of a "throw the popcorn and root for everyone except Alan Rickman in his storyline and love Liam Neeson's kid in his" movie, so I was really looking forward to showing my new partner this movie. I told him this, he agreed, we set a date to have a hot-cocoa-and-movie date.
A few days before this scheduled date, his other new partner of less than a year, who was much more demonstrative on social media than I was/am, posted about all of the romantic wintery things they had done over the weekend, including watching Love Actually. I asked my partner about this, he said he hadn't picked the movie they watched and that consuming media is different with each individual so it was totally fine with him to watch it twice in a week, so he was still looking forward to watching it with me. But I was really bothered. I'd set a specific date for this. I'd said it was one of my favorites and important to me, and made it clear that I didn't have a lot of holiday-season expectations but this was one of them. It wasn't that he'd seen it a year ago, or months before, or seen it with me and then chosen to re-watch: he'd made a plan, and then done the plan with someone else less than a week before. It made me feel suddenly not-special and sickeningly jealous and sad in a way I hadn't really experienced in polyamory prior to that. (I'd had my moments of envy or insecurity, in the years prior, but none of them had been "I feel invisible because my request for something special was heard, acknowledged, and ignored.")
We figured out another plan to do that night - honestly, I don't remember what we substituted in, just that it involved going out instead and some Christmastime tradition of his - and I did my Love Actually re-watch with a friend instead; because it felt very uncomfortable and "seconds" for me to repeat a date he'd just been on so soon. Today I think I'd just ask someone to give me a heads up so I didn't learn about the situation from social media, and I'd be ok with "let's see the same movie/go to the same cool place/do the same interactive exhibit you saw with Partner Y" to a much greater degree, just from years of unpacking the underlying feelings that made that uncomfortable, but maybe not if it was my favorite or something important to me.
Here's the lesson that comes out of all of this. Knowing which things are important to you and which ones of those are things you want to unpack cultural norms about "not sharing" around vs which ones you want to "keep special" for a given relationship is a very individual thing to you, and to each of your relationships. If it's super different in each relationship, there may be questions about why from your partners that you want to work on figuring out the answers to - but also, it's okay if that's the case. We're all human and our relationships grow and change at different speeds, and so does our knowledge of what matters to us. I would never get mad about someone letting someone else show them Love Actually today, but if they hadn't seen the Clue movie and made that plan with me and then watched it with a friend or partner in the space between making and executing the plan? I'm very frustrated about that, for all of time. Maybe yours is a particular activity or hobby you took up together and you don't mind if other new partners are also involved but you want a heads up before they become the +1 to board game night, or you want the option to all go together to a convention, or whatever. Only you can say what yours is (mine is petty, I know - like I said, I'm only human). But identify it or them and let your partner know, and try to keep it to a couple things, not a laundry list, and you'll be fine.