Ordered NSCountedSet. Subclassing and overwrite `- (id)keyForObject:(id)object` to specify your own keys for objects.

PCCountedObjectContainer.h

//
//  PCCountedObjectContainer.h
//  Slingshot
//
//  Created by Maximilian Mackh on 02/02/14.
//  Copyright (c) 2014 Professional Consulting & Trading GmbH. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface PCCountedObjectContainer : NSObject

- (instancetype)initWithContainer:(PCCountedObjectContainer *)container;

- (void)addObject:(id)object;
- (void)insertObject:(id)object atIndex:(NSInteger)index;

- (void)removeObject:(id)object;

- (void)addAllObjectsFromContainer:(PCCountedObjectContainer *)container;
- (void)removeAllObjectsFromContainer:(PCCountedObjectContainer *)container;

- (NSInteger)countForObject:(id)object;

- (NSArray *)allObjects;
- (NSInteger)count;

@end

PCCountedObjectContainer.m

//
//  PCCountedObjectContainer.m
//  Slingshot
//
//  Created by Maximilian Mackh on 02/02/14.
//  Copyright (c) 2014 Professional Consulting & Trading GmbH. All rights reserved.
//

#import "PCCountedObjectContainer.h"

typedef NS_ENUM(NSInteger, PCCountedObjectMethod)
{
    PCCountedObjectMethodAdd,
    PCCountedObjectMethodRemove
};

@implementation PCCountedObjectContainer
{
    NSMutableDictionary *_objectCounter;
    NSMutableArray *_objects;
}

- (instancetype)init
{
    self = [super init];
    if (!self) return nil;
    
    _objectCounter = [NSMutableDictionary new];
    _objects = [NSMutableArray new];
    
    return self;
}

- (instancetype)initWithContainer:(PCCountedObjectContainer *)container
{
    self = [self init];
    if (!self) return nil;
    
    NSInteger index = 0;
    for (id object in container.allObjects)
    {
        NSInteger cycles = [container countForObject:object];
        while (cycles --> 0)
        {
            id targetObject = [container.allObjects objectAtIndex:index];
            [self addObject:targetObject];
        }
        index++;
    }
    
    return self;
}

- (void)addObject:(id)object
{
    [self modifyMappingForObject:object method:PCCountedObjectMethodAdd index:NSNotFound];
}

- (void)addAllObjectsFromContainer:(PCCountedObjectContainer *)container
{
    for (id object in [container allObjects])
    {
        NSInteger cycles = [container countForObject:object];
        while (cycles --> 0)
        {
            [self addObject:object];
        }
    }
}

- (void)removeAllObjectsFromContainer:(PCCountedObjectContainer *)container
{
    for (id object in [container allObjects])
    {
        NSInteger cycles = [container countForObject:object];
        while (cycles --> 0)
        {
            [self removeObject:object];
        }
    }
}

- (void)insertObject:(id)object atIndex:(NSInteger)index
{
    [self modifyMappingForObject:object method:PCCountedObjectMethodAdd index:index];
}

- (void)removeObject:(id)object
{
    [self modifyMappingForObject:object method:PCCountedObjectMethodRemove index:NSNotFound];
}

- (void)modifyMappingForObject:(id)object method:(PCCountedObjectMethod)method index:(NSInteger)index
{
    id key = [self keyForObject:object];
    
    if (_objectCounter[key])
    {
        NSInteger counter = [_objectCounter[key] integerValue];
        counter = (method == PCCountedObjectMethodAdd) ? (counter + 1) : (counter - 1);
        _objectCounter[key] = @(counter);
    }
    
    if (method == PCCountedObjectMethodAdd && !_objectCounter[key])
    {
        _objectCounter[key] = @(1);
        
        if (index == NSNotFound)
        {
            [_objects addObject:object];
        }
        else
        {
            [_objects insertObject:object atIndex:index];
        }
        
        return;
    }
    
    if (method == PCCountedObjectMethodAdd && _objectCounter[key])
    {
        if (index != NSNotFound)
        {
            [_objects removeObject:object];
            [_objects insertObject:object atIndex:index];
        }
    }
    
    if (method == PCCountedObjectMethodRemove && ![_objectCounter[key] integerValue])
    {
        [_objectCounter removeObjectForKey:key];
        [_objects removeObject:object];
    }
}

- (NSInteger)countForObject:(id)object
{
    id key = [self keyForObject:object];

    if (_objectCounter[key])
    {
        return [_objectCounter[key] integerValue];
    }
    
    return 0;
}

- (NSArray *)allObjects
{
    return _objects;
}

- (NSInteger)count
{
    return _objects.count;
}

- (id)keyForObject:(id)object
{
    return @([object hash]).stringValue;
}

@end