Deduplicate users in group notifications

Changelog-Fixed: Deduplicate users in notifications
Closes: #1326

damus/Util/DisplayName.swift

@@ -38,7 +38,7 @@ enum DisplayName {
 
 
 func parse_display_name(profile: Profile?, pubkey: String) -> DisplayName {
-    if pubkey == "anon" {
+    if pubkey == ANON_PUBKEY {
         return .one(NSLocalizedString("Anonymous", comment: "Placeholder display name of anonymous user."))
     }
     

damus/Util/Keys.swift

@@ -9,6 +9,7 @@ import Foundation
 import secp256k1
 
 let PUBKEY_HRP = "npub"
+let ANON_PUBKEY = "anon"
 
 struct FullKeypair: Equatable {
     let pubkey: String