Multiple lightbox opens at one click

We use this plugin at an website. When we click at the lightbox link for the first time one lightbox opens. When we click for the second time at the same link (without reloading the page) the lightbox opens twice. When we click for the third time at the link without reloading the lightbox opens three times. etc.

This seems not to be correct. When i have clicked for the second time of more, it think it always must opens one lightbox.

What goes wrong?

http://wordpress.org/plugins/lightbox-plus/