Just let it go. . . :^) it’s out of your control— so, just say “heck with it!” Or post it anyway, as who cares if it’s a “repeat?” As you can always do so in a new and original way (that being your own!) : )
What sort of things are you thinking of posting that someone else already posts? If you are posting your original thoughts, even if someone else has similar thoughts, the wording is your own, and can be appreciated independently of appreciating other people's posts. For example on this site, I'm sure you know since you have a lot of posts, 30 people might respond supportively to someone's thread, many saying similar things, but I don't think that means the later posts have any less meaning.
Or are you talking about posting a link sharing someone else's idea/work, in which case there is less reason to re-post it after someone else shared it?
What do you mean by you don't like it when the other people post?
That's very true. We arrive at ideas independently, but very few of them are ever brand new or 'original' thoughts. What's important is one's perspective on it, which is much more likely to be unique and interesting.