Google landed me here because I also thought this was a bug. Has anyone contacted support about this? My issue is, regardless of workflow, not even the documentation mentions the step that jimc pointed out above.
http://www.simplify3d.com/support/tutor ... ur-models/
doesn't mention this at all.
http://www.simplify3d.com/support/tutor ... -printing/
mentions selecting the models, but it's having you add a new process, and doesn't mention that this step won't happen if you already have a process added.
I do agree though, it would be straightforward to have the software check to see if there's a process already added and attached to the model, and to just duplicate that attachment step for the duplicated models. If they're worried that that "automates things too much" they could add an alert prompt informing the user that has happened (and for UX sake, put a checkbox option to not show that again).