Duplicate symbol link error for iOS

Question

I'm new in the world of iOS development, but I've previously done some limited C and C++ programming, which makes me believe that my Objective-C code is correct, even though Xcode is giving me the following Linker error:

Ld /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Products/Debug-iphoneos/objc-vvc.app/objc-vvc normal armv7
    cd /Users/nillerr/Documents/xcode/objc-vvc
    export IPHONEOS_DEPLOYMENT_TARGET=8.1
    export PATH="/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin:/Applications/Xcode.app/Contents/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch armv7 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS8.1.sdk -L/Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Products/Debug-iphoneos -F/Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Products/Debug-iphoneos -filelist /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/objc-vvc.LinkFileList -Xlinker -rpath -Xlinker @executable_path/Frameworks -dead_strip -fobjc-arc -fobjc-link-runtime -miphoneos-version-min=8.1 -Xlinker -dependency_info -Xlinker /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/objc-vvc_dependency_info.dat -o /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Products/Debug-iphoneos/objc-vvc.app/objc-vvc

duplicate symbol _OBJC_IVAR_$_VidyoOutEvent._event in:
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/AppVidyoClient.o
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/LoginViewController.o
duplicate symbol _OBJC_IVAR_$_VidyoOutEvent._param in:
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/AppVidyoClient.o
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/LoginViewController.o
duplicate symbol _OBJC_IVAR_$_VidyoOutEvent._paramSize in:
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/AppVidyoClient.o
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/LoginViewController.o
duplicate symbol _OBJC_IVAR_$_VidyoOutEvent._data in:
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/AppVidyoClient.o
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/LoginViewController.o
duplicate symbol _OBJC_METACLASS_$_VidyoOutEvent in:
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/AppVidyoClient.o
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/LoginViewController.o
duplicate symbol _OBJC_CLASS_$_VidyoOutEvent in:
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/AppVidyoClient.o
    /Users/nillerr/Library/Developer/Xcode/DerivedData/objc-vvc-bnuejnodyndpcggqtwhovkyuuivi/Build/Intermediates/objc-vvc.build/Debug-iphoneos/objc-vvc.build/Objects-normal/armv7/LoginViewController.o
ld: 6 duplicate symbols for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I've already searched and tried numerous solutions, including editing the project file, checking the Build Phases for duplicates or .h files, but nothing has worked so far. How can I solve this problem?

I've attached the code for the files in question:

AppVidyoClient.h

#ifndef vvc_AppVidyoClient_h
#define vvc_AppVidyoClient_h

#import "VidyoClient.h"

@interface AppVidyoClient : NSObject <VidyoClient>
{

}

+ (AppVidyoClient*) instance;

@end

#endif

AppVidyoClient.m

#import <Foundation/Foundation.h>
#import "AppVidyoClient.h"
#import "NSURLConnection+Blocks.h"
#import "Tolo.h"
#import "Messages.h"

static AppVidyoClient *instance;

@interface AppVidyoClient ()
{

}

@end

@implementation AppVidyoClient
#pragma mark Class methods
+ (AppVidyoClient *) instance
{
    if (!instance) {
        instance = [[AppVidyoClient alloc] init];
    }
    return instance;
}

#pragma mark init / dealloc
- (id) init
{
    if (self = [super init]) {

    }

    return self;
}

- (void) login:(NSString *)portal username:(NSString *)username password:(NSString *)password
{
    NSDictionary *params = @{
                             @"portal": portal,
                             @"username": username,
                             @"password": password
                             };

    [self post:@"login" parameters:params];
}

- (void) post:(NSString *)action parameters:(NSDictionary *)parameters
{
    NSString *url = [NSString stringWithFormat:@"http://webadmin.lifeline.viewcare.com/ios.php?do=%@", action];
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:url]];
    [request setHTTPMethod:@"POST"];
    [request setValue:@"text/json" forKey:@"Content-Type"];

    NSError *error;
    NSData *body = [NSJSONSerialization dataWithJSONObject:parameters options:0 error:&error];

    [request setHTTPBody:body];

    NSString *strLength = [NSString stringWithFormat:@"%lu", (unsigned long)[body length]];
    [request setValue:strLength forKey:@"Content-Length"];

    [NSURLConnection asyncRequest:request success:^(NSData *data, NSURLResponse *response) {
        [self onSuccess:data response:response];
    } failure:^(NSData *data, NSError *error) {
        [self onFailure:data error:error];
    }];
}

- (void) onSuccess:(NSData *)data response:(NSURLResponse *)response
{
    NSError *error;
    NSDictionary *resp = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];

    VidyoOutEvent *event = [[VidyoOutEvent alloc] init];
    event.event = (VidyoClientOutEvent) [resp valueForKey:@"event"];
    event.param = (__bridge VidyoVoidPtr)([resp valueForKey:@"param"]);
    event.paramSize = (int) [resp valueForKey:@"paramSize"];
    event.data = (__bridge VidyoVoidPtr)([resp valueForKey:@"data"]);

    PUBLISH(event);
}

- (void) onFailure:(NSData *)data error:(NSError *)error
{

}

@end

LoginViewController.h

#ifndef vvc_LoginViewController_h
#define vvc_LoginViewController_h

#import <UIKit/UIKit.h>

@interface LoginViewController : UIViewController

@property (weak, nonatomic) IBOutlet UITextField *mTxtServer;

- (IBAction)onSignInClick:(UIButton *)sender;

@end

#endif

LoginViewController.m

#import <UIKit/UIKit.h>
#import "Tolo.h"
#import "LoginViewController.h"
#import "VidyoClient.h"
#import "AppVidyoClient.h"
#import "AppDelegate.h"
#import "Messages.h"

#define SUBSCRIBE_TO(_event_id_) - (void) onVidyoOutEvent:(VidyoOutEvent *) event data:(VidyoClientOutEvent) data

@interface LoginViewController ()
{
    // Private members
    int mPrivateInt;
}

@property (readwrite, nonatomic) id <VidyoClient> client;
@end

@implementation LoginViewController
#pragma mark Class Methods

#pragma mark Init & Dealloc methods
- (id) init
{
    if (self = [super init]) {

    }

    return self;
}

#pragma mark ViewController
- (void) viewDidLoad
{
    [super viewDidLoad];
    _client = AppVidyoClient.instance;

    // Register on EventBus
    REGISTER();
}

#pragma mark EventBus handlers
SUBSCRIBE(VidyoOutEvent)
{
    switch (event.event) {
        case VIDYO_CLIENT_OUT_EVENT_SIGNED_IN:
        {
            NSString *param = event.param;
            if ([param isEqualToString:@"OK"]) {
                // Move to signed in view
                [self performSegueWithIdentifier:@"signInComplete" sender:self];
            }
            break;
        }
    }
}

#pragma mark Instance methods
- (IBAction)onSignInClick:(UIButton *)sender {
    NSString *portal = _mTxtServer.text;
    [_client login:portal username:@"" password:@""];
}

@end

Messages.h

#ifndef objc_vvc_Messages_h
#define objc_vvc_Messages_h

typedef enum VidyoClientOutEvent_
{
    VIDYO_CLIENT_OUT_EVENT_SIGNED_IN = 2303,
} VidyoClientOutEvent;

typedef void *VidyoVoidPtr;
typedef unsigned int VidyoUint;

@interface VidyoOutEvent : NSObject

@property VidyoClientOutEvent event;
@property VidyoVoidPtr param;
@property VidyoUint paramSize;
@property VidyoVoidPtr data;

@end

@implementation VidyoOutEvent

@end

#endif

Answer 1

I must have been blinded by the new syntax of Objective-C... The solution is rather simple:

Move

@implementation VidyoOutEvent

@end

Out of Messages.h and into a implementation file Messages.m

#import "Messages.h"

@interface VidyoOutEvent ()
{

}
@end

@implementation VidyoOutEvent

@end