Are banks allowed to take charges from your account, knowing there are DD's to come out + making you overdrawn in the process, again creating a further charge for themselves to take the following month?
Yes they are. They are not obliged to check whether you have any DD's due, it is upto you to make sure you have sufficient funds in to cover the direct debits.
The bank doesn't know you have DDs due (unless they are already being claimed - ie the payee has asked for that week/month's payment.) It isn't capable of working out that because something went out on the 26th of last month it will go out again on the 26th of this month - it may be a different day or not taken at all.