Consider the interaction $$e^-+e^- \rightarrow e^-+e^-$$ The following is a tree level Feynman diagram for this: We can also make the paths of the two electrons on the right hand side cross over and this is a distinct diagram.
However lets consider the interaction $$e^-+e^+ \rightarrow e^-+e^+$$ The above diagram would be a tree level diagram in this situation (swap the top electron for a positron). However I am led to believe that in this case making the path of the electron and positron on the right hand side cross would not give a distinct diagram.
Why is this the case and generally when does a 'cross-over' give rise to a distinct diagram? Thanks :)