Adding custom defined objects to NSMutableArray overwrite the whole array

-(id) initWithData:(NSString *)lastName: (NSString *)firstName{
self->firstname =  firstName;
self->lastname =  lastName;

return self;
}


-(void) printData {
NSLog(@"firstname: %@", firstname);
NSLog(@"lastname: %@", lastname);
}

so whenever I create a new object using the above init function. And Add objects to a NSMutableArray, using the addObject function.

NSMutableArray *objectArray = [[NSMutableArray alloc] init];

CustomObject *tempObject = [[CustomObject alloc] initWithData: @"smith": @"john"];
CustomObject *tempObjectb = [[CustomObject alloc] initWithData: @"brown": @"william"];
[objectArray addObject:tempObject];
[objectArray addObject:tempObjectb];

[[objectArray objectAtIndex:0] printData];
[[objectArray objectAtIndex:1] printData];

objects at index 1, and 0 always equal the whichever object was added to the array last.
This also happens if I use a for loop, or have more than 2 objects, all values when printed, turn to the values of the last added object to the objectArray. Let me know if there is any information that I am missing. Is there something that I am missing?