Unwanted renaming while copy/pasting in a duplicate tab... why does it rename?

I have 2 tabs open - 1 where I copy from, the other where I paste inside. Both under the same project.
When I paste what I just copied in the other tab, I get a message saying: " For pasting cross-site we had to remove CMS bindings. Renamed “Collection Item 2” class to “Collection Item 3” in order to avoid conflicts."

Why does it rename? I thought it is possible to copy/paste between pages as long as I am on the same project.

Any ideas on how to avoid renaming?

Thanks in advance!

Here is my site Read-Only: LINK
(how to share your site Read-Only link)