Maybe this is too hard to implement, but I often find myself having the same idea twice (or remembering to do something twice, but not remembering whether I already entered it) so I've gotten into the habit of searching for some keywords first, before adding the task.

What I'd prefer to do is just start typing in the name of the task, and have it suggest or autocomplete using the names of other tasks. This could also help when trying to add a subtask, or when you want to add a subtask to a task that is not in view (and sometimes, you have no idea where it is, so you have to search for it first).