TaskModel: Do not duplicate items

When inserting new items into the TaskFilterModel we could end up
duplicating the items. Do not do that.

Change-Id: I2450bac185a881e16f23368b39ce5e273c061d99
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>