Pictures doubled in folder..?

Not sure how it happened, but I cannot for the life of me figure out how to get them back to single copies. Say for example, I had 500 pictures on my phone, I plugged it up to the computer and when I went to eject it from the usb port, I checked my phone picture folder and there was 1,000 pictures. I am afraid to delete either copy in fear of deleting the original one. Does anybody have any idea or information on this and why it happens?