Attempt to handle tricky situation where some instances of an online account contain the nonce and recent block signature, whereas other instances do not (due to being sent via an older peer).

Right now, two OnlineAccountData objects are considered equal if they have matching timestamps, signatures, and public keys. This reduces the chance of multiple versions of the same online account data from being sent around the network. The downside is that an instance containing a nonce value can be ignored due to already having an inferior OnlineAccountData instance in the list.

The current approach is this:
- Only allow new duplicate onlineAccountData to be added to the import queue if it's superior to the one we already have.
- Remove the existing, inferior data at the time of import (once the new data is considered valid).

This is only a temporary problem, and can be simplified once the additional fields in OnlineAccountsV3Message become required rather than optional.

src/main/java/org/qortal/controller/OnlineAccountsManager.java

@@ -231,10 +231,53 @@ public class OnlineAccountsManager extends Thread {
                 LOGGER.trace(() -> String.format("Added online account %s with timestamp %d", otherAccount.getAddress(), onlineAccountData.getTimestamp()));
             }
 
+            // Remove existing version of this online account data if the new one is superior
+            if (isOnlineAccountsDataSuperior(onlineAccountData)) {
+                this.onlineAccounts.remove(onlineAccountData);
+            }
+
             this.onlineAccounts.add(onlineAccountData);
         }
     }
 
+    /**
+     * Check if supplied onlineAccountData is superior (i.e. has a nonce value) than existing record.
+     * Two entries are considered equal even if the nonce and block signature differ, to prevent
+     * multiple variations co-existing. For this reason, we need to be able to check
+     * if a new OnlineAccountData should replace the existing one, which may be missing the nonce.
+     * @param onlineAccountData
+     * @return
+     */
+    private boolean isOnlineAccountsDataSuperior(OnlineAccountData onlineAccountData) {
+        if (onlineAccountData.getNonces() == null || onlineAccountData.getNonces().isEmpty()) {
+            // New online account data has no nonce value(s), so it won't be better than anything we already have
+            return false;
+        }
+
+        // New online account data has nonce value(s), so we need to check if the existing one does
+        OnlineAccountData existingOnlineAccountData = null;
+        for (OnlineAccountData acc : this.onlineAccounts) {
+            if (acc.equals(onlineAccountData)) {
+                // Found existing online account data
+                existingOnlineAccountData = acc;
+                break;
+            }
+        }
+
+        if (existingOnlineAccountData == null) {
+            // No existing online accounts data, so nothing to compare
+            return false;
+        }
+
+        if (existingOnlineAccountData.getNonces() == null || existingOnlineAccountData.getNonces().isEmpty()) {
+            // Existing data has no nonce value(s) so we want to replace it with the new one
+            return true;
+        }
+
+        // Both new and old data have nonce values so the new data isn't considered superior
+        return false;
+    }
+
     public void ensureTestingAccountsOnline(PrivateKeyAccount... onlineAccounts) {
         if (!BlockChain.getInstance().isTestChain()) {
             LOGGER.warn("Ignoring attempt to ensure test account is online for non-test chain!");
@@ -648,7 +691,12 @@ public class OnlineAccountsManager extends Thread {
 
             // Do we already know about this online account data?
             if (onlineAccounts.contains(onlineAccountData)) {
-                continue;
+
+                // Don't import if it's no better than the one we already have
+                if (!isOnlineAccountsDataSuperior(onlineAccountData)) {
+                    // Do NOT remove the existing online account data - this takes place after validation
+                    continue;
+                }
             }
 
             // Is it already in the import queue?