Hi,
I tried to copy/paste an Activiti diagram with Eclipse Designer 5.8 running in Spring Tools Suite v2.8.0.RELEASE
The new diagram appears as expected in the project tree but whenever I try to open it, it opens the "original" diagrams (the one I copied from) instead! (yes, I click on "myNewDiagram.activiti" and it opens a new tab with "myOldDiagram.activiti" instead of the expected file.
I worked around this by manually re-creating my diagram from scratch instead of copying and modifying an existing diagram.