iPhoto is assigning duplicate image numbers to different photos

Upon importing photos from my iPhone (into my Mac) iPhoto is assigning duplicate file names; thus it asks if I want to replace the old photo with the new one even though they are totally different images, shot at different months, uploaded on different dates.

I use my iPhone exclusively to take photos, so it's not a camera issue (I don't own a camera).

I don't want to delete/replace ANY of the existing photos with the new images.

Help. :/