Back on Navigation Controller causes Collection View items to duplicate - ios

Question

I think this is probably a simple fix, but I am struggling to find the right solution
I have the following:
ContactList - a UICollectionView that loads the content with viewDidAppear: method. When a item is selected in the collection view I fire this method:
[self performSegueWithIdentifier:#"pushContactDetail" sender:self];
So far so good. Everything works great. However when I navigate back from the ContactDetail page using the Navigation Controller button my ContactList duplicates the content.
Is there something I should be doing to prevent the viewDidAppear: from running again?
I don't think I want to set the collection to nil when I push the ContactDetail page as that would cause the content to be reloaded each time...
Here is the code:
-(void) viewDidAppear
{
[super viewDidAppear];
[self.view setBackgroundColor:myColorLightGrey];
_contactList = [[NSMutableArray alloc] init];
[self displayLoadingAndDisableTableViewInteractions];
[self queryData];
}
- (void) queryData
{
//Find the Data
PFQuery *query = [PFUser query];
PFUser *consumer = [PFUser currentUser];
[query includeKey:User_followers];
[query whereKey:#"email" equalTo:consumer.email];
query.maxCacheAge = 60 * 60 * 24; // One day, in seconds
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error)
{
for (PFUser *tmpConsumer in objects)
{
for (PFUser *publisher in [tmpConsumer objectForKey:User_followers])
{
[_contactList addObject:publisher];
}
}
[_collectionView reloadData];
[self hideLoadingAndEnableTableViewInteractions];
}];
}
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"contactCell";
ContactCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];
//SNIPPED REMAINDER OF CODE
...
}

Answer 1

In your queryData method, nil out the array first
_contactList = nil;
_contactList = [NSMutableArray array]; // or [[NSMutableArray alloc] init];
Move the alloc/init method for _contactList like this:
- (void) queryData
{
_contactList = nil;
_contactList = [[NSMutablArray alloc] init];
//Find the Data
PFQuery *query = [PFUser query];
PFUser *consumer = [PFUser currentUser];
[query includeKey:User_followers];
[query whereKey:#"email" equalTo:consumer.email];
query.maxCacheAge = 60 * 60 * 24; // One day, in seconds
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error)
{
for (PFUser *tmpConsumer in objects)
{
for (PFUser *publisher in [tmpConsumer objectForKey:User_followers])
{
[_contactList addObject:publisher];
}
}
[_collectionView reloadData];
[self hideLoadingAndEnableTableViewInteractions];
}];
}

Answer 2

Navigate back with Animation true. It will solve your problem.