Fixes : Same Group can be added twice to a playlist #2440

#2440 ,

Now, same group can't be dropped twice on a playlist entry.
@aronasorman , we have collection of models (of group type) so, pluck( 'id') , goes to every model one by one in the collection and fetch their id and push it into an array.
So, groups_assigned.pluck("id") returns us an array of all the group id's we have in that collection means all those groups which are already assigned. Say this array A.
group.get("id") returns us the id of the group which we are currently trying to assign.
So when i check this group id in the above returned array A using the indexOf method and this method will return -1 if the group id is not present in array A.
That means we should assign the group, and if its already present then we should not.