You've just posted your Tropetastic masterpiece on TLP, it's witty, it's Genre Savvy, and with a bit of polish and some more examples it's about ready to go. Then along comes a fellow Troper who brusquely informs you that your trope is basically identical to another and wishes to point out that Yes, We Do Have This One.
Except...this time, it actually isn't. You guess what they've done - they've only skimmed the title or laconic, or maybe the first sentence or two of the definition, and got the wrong first impression (which is what reminded them of another trope). Maybe they just made a snap decision that Bullet Time is identical to HammerTime because they share one word of the title. Or that Slippy-Slidey Ice World and Kill It with Ice are the same because they both involve ice. They may even dismiss it as People Sit on Chairs simply because they haven't taken the time to grasp what you mean (though to be fair, consider if you made a mistake before linking them here).
There are plenty of those who are quick to write-off new tropes, particularly on TLP, by claiming that they are identical to one (or more!) that have been previously established, and do so without taking the time to read and understand one or both of their descriptions (and thus compare them erroneously). If this is the case, tell them to Please Read That Again — even though you know they didn't the first time (but hell, we have to be polite to the little crabsacks, right?).
If you have been linked to this page, it is likely that the trope author thinks that your comparison is hasty and very obviously wrong, and that the two are simply not comparable (though they themselves should read both to make sure!).
Note to trope creators: please think carefully and take time to compare the tropes yourself before linking to this page! If you have read both thoroughly and they are very, very, definitely not Ridiculously Similar Tropes then go ahead, but bear in mind that they may be reacting to a new trope that is simply too unclear, or too poorly worded, to understand (don't despair! TLP is just the first draft!). In all cases, please ask for clarification before making comparisons.
Also note: People mistakenly claiming your trope idea is identical to one we have is often a sign that your laconic was badly worded — try rewriting it to give a more accurate first impression.