It is is showing twice as many, then you probably still have them all on the phone itself. Or, you have them in your Google account. Either will show as a second copy. To make it just the one, and to clear up space, you would have to delete any copies that are on the phone and not the SD card.
Now if it isn't ones on the phone, but on your google account, then you need to change the settings in the Gallery on the phone to only show photos that are on the SD card. You don't want to do that in the case of copies being on the phone, because then the photos are still on the phone (so you have two copies) just not listed.