Does it show up on your Select Aircraft menu?
If it does, it might have an aircraft folder with an unrecognizable name. In that case, open your aircraft folder and View Details. Sort by Date Modified and that folder will show up at the top of the list. Then you can rename it with a name you'll recognize in the future.
If it doesn't show on your Select Aircraft menu, then you must be using an auto installer and letting it run in fully automatic mode. This is just one example of what can happen when you use those things that way. Try Nigel's suggestion, and let this be a lesson to you!
If it bugs you that there's an extra, unused copy of the aircraft cluttering up your hard drive, you can use Windows' Search feature to find some file with a name unique to that plane, such as the AIR file. Then select Open Containing Folder and delete the extra copy. Since you must have gotten a correct installation in order to find such a file name, your Search will turn up two copies of that file. One, of course, will be right where it belongs, in an aircraft folder within the main Aircraft folder. Make sure you delete the folder that contains the other one!