Let's say that you just realized that sexy girls with parasols are common in fiction. Hooray, a new trope! You quickly go to the Trope Launch Pad and suggest "Sexy Parasol" as a new trope... only to get turned down by someone who points out that we already have Parasol of Prettiness.
But wait! There are some subtle differences between "sexiness" and "prettiness". Technically, you could still launch yours as a Sister Trope. Right?
If you got this Predefined Message, someone wants to remind you that Tropes Are Flexible. We are discovering more tropes because we want to cover every storytelling device in the media. Cover, not categorize. Splitting and organizing them as Subtropes, Supertropes and Sister Tropes are just the means to make the browsing more comfortable and practical; but this doesn't mean that every single thing that can be identified as a distinct trope should get its own page. What about "Parasol of Allure"? What about "Parasol of Immaculate Fashion Sense"? A separate trope for when the parasol actually gets used to block out the sun, and for when it's purely employed for the aesthetic effect? Where does it stop?
You might say that it doesn't, and yes, we should have every single conceivable trope. That's missing the forest for the trees. There's something to be gained from Lumping all the examples of a parasol connected with sexiness, or beauty, or what-have-you, in the same place and have them add up to something. This is the whole idea of what a "trope" is; otherwise, every single thing that ever happened in fiction might as well be its own trope.
And that problem just arises from considering the issue in theory; it becomes a whole lot messier in practice. Suppose we were to launch "Sexy Parasol"? We would have to note the subtle differences on both pages, people would still mix them up, it would end up at the Trope Repair Shop, our repairmen would argue about the difference between "sexiness" and "prettiness" for twenty pages, and someone's mother would get compared to Hitler. And all this just to split a trope that doesn't have that many examples to begin with.
We don't have a hard rule about the minimum difference two tropes must have; as long as there is some difference, technically you are allowed to launch it. But if consensus has it that your trope is Ridiculously Similar to some other trope, it may be the time to take a step back and rework your suggestion to focus on the really essential parts so it becomes more distinct. If that still doesn't help, that's nothing to fret about - in fact, it's good news. It means that Yes, We Do Have This One. Of course, before linking to that page or this one you should make sure that you yourself have read and understood the main body text of both tropes. If not, Please Read That Again.