Apply assert to ensure singletons are only created once.

// FREEBIE

Signal/src/contact/OWSContactsManager.m

@@ -41,6 +41,8 @@ NSString *const OWSContactsManagerSignalRecipientsDidChangeNotification =
     _latestContactsById = @{};
     _avatarCache = [NSCache new];
 
+    OWSSingletonAssert();
+
     return self;
 }
 

Signal/src/environment/Environment.m

@@ -52,6 +52,8 @@ static Environment *environment = nil;
     _networkManager = networkManager;
     _messageSender = messageSender;
 
+    OWSSingletonAssert();
+
     return self;
 }
 

Signal/src/environment/NotificationsManager.m

@@ -37,6 +37,8 @@
     NSURL *newMessageURL = [[NSBundle mainBundle] URLForResource:@"NewMessage" withExtension:@"aifc"];
     AudioServicesCreateSystemSoundID((__bridge CFURLRef)newMessageURL, &_newMessageSound);
 
+    OWSSingletonAssert();
+
     return self;
 }
 

Signal/src/environment/PropertyListPreferences.m

@@ -29,6 +29,18 @@ NSString *const PropertyListPreferencesKeyHasDeclinedNoContactsView = @"hasDecli
 
 @implementation PropertyListPreferences
 
+- (instancetype)init
+{
+    self = [super init];
+    if (!self) {
+        return self;
+    }
+
+    OWSSingletonAssert();
+
+    return self;
+}
+
 #pragma mark - Helpers
 
 - (void)clear {

Signal/src/util/Pastelog.m

@@ -103,6 +103,8 @@
 -(instancetype)init {
     if (self = [super init]) {
         self.responseData = [NSMutableData data];
+
+        OWSSingletonAssert();
     }
     return self;
 }