The original may have perished. It's an odd thing to double up though, because the cap can be removed easily and is a good way to check the lubricant and insert a bit more grease. If it's not recoverable, order a couple of new ones, they secure these days with cable ties, and take no time to fit. You don't need to remove the wheels for this. If you put the rack on lock so the end protrudes, you'll see how much lubricant is in there.
