K2 add item display twice in front end

I am not 100% sure I follow what you are trying to do here.  "When you say when you try to edit or add an item" What are you referring to?  Can you provide more details...

Thanks

change line 62 in the modal.css file. media/system/css/modal.css

.sbox-content-image#sbox-content {
overflow: visible;
}

to

.sbox-content-image#sbox-content {
overflow: hidden;
}