On a second thought... It is better to merge all proposals to the original project idea, where most people interested is subscribed.

@Qgil, but as all proposals were a subtask of the original project ideas, doesn't it make more sense to close them as "Declined"?
As far as I understand, merging tasks only merges the subscribers to the task. And creating a subtask also maintains the same subscribers as the parent task. Going by that, the subscribers for the proposals and the original task should be the same (unless someone removed themselves).

The difference is just a detail. Marking all proposals as duplicates of the project idea (owned now by the intern accepted) is a way to indicate that all the energy in the proposals (people and good ideas) are now concentrated in one task. Declining sounds more to 'we are not going to do this'. We will do it, but in a different implementation.

Also note that some of the proposals were not created as subtasks, and also that there is the possibility of people subscribing to the subtask after its creation, without subscribing to the main task.

All in all, I still think it is nicer and more practical to mark proposals as duplicates.

@Qgil, this is done now. How should I go about cleaning Possible-Tech-Projects ? Or should we just let it be until the next round approaches?

Looks good to me! Let it be, yes.