Previous commit gave duplicates. Lets try this.

src/com/massivecraft/factions/entity/Faction.java

@@ -29,6 +29,7 @@ import com.massivecraft.factions.util.RelationUtil;
 import com.massivecraft.massivecore.Named;
 import com.massivecraft.massivecore.collections.MassiveList;
 import com.massivecraft.massivecore.collections.MassiveMapDef;
+import com.massivecraft.massivecore.collections.MassiveSet;
 import com.massivecraft.massivecore.collections.MassiveTreeSetDef;
 import com.massivecraft.massivecore.comparator.ComparatorCaseInsensitive;
 import com.massivecraft.massivecore.mixin.MixinMessage;
@@ -1008,7 +1009,8 @@ public class Faction extends Entity<Faction> implements EconomyParticipator, Nam
 	// FOREIGN KEY: MPLAYER
 	// -------------------------------------------- //
 	
-	protected transient List<MPlayer> mplayers = new ArrayList<MPlayer>();
+	protected transient Set<MPlayer> mplayers = new MassiveSet<MPlayer>();
+	
 	public void reindexMPlayers()
 	{
 		this.mplayers.clear();

src/com/massivecraft/factions/entity/MPlayer.java

@@ -33,7 +33,6 @@ import com.massivecraft.massivecore.util.MUtil;
 import com.massivecraft.massivecore.util.Txt;
 import com.massivecraft.massivecore.xlib.gson.annotations.SerializedName;
 
-
 public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipator
 {
 	// -------------------------------------------- //
@@ -89,33 +88,41 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 	// UPDATE FACTION INDEXES
 	// -------------------------------------------- //
 
-	public void updateFactionIndexes(Faction before, Faction after)
+	public void updateFactionIndexes(String beforeId, String afterId)
 	{
+		// Really?
+		if (!Factions.get().isDatabaseInitialized()) return;
+		if (!this.attached()) return;
+
+		// Fix IDs
+		if (beforeId == null) beforeId = MConf.get().defaultPlayerFactionId;
+		if (afterId == null) afterId = MConf.get().defaultPlayerFactionId;
+
 		// NoChange
-		if (MUtil.equals(before, after)) return;
+		if (MUtil.equals(beforeId, afterId)) return;
 
-		// Before
+		// Resolve
+		Faction before = Faction.get(beforeId);
+		Faction after = Faction.get(afterId);
+
+		// Apply
 		if (before != null) before.mplayers.remove(this);
-		
-		// After
 		if (after != null) after.mplayers.add(this);
 	}
 
 	@Override
 	public void postAttach(String id)
 	{
-		if (!Factions.get().isDatabaseInitialized()) return;
+		String beforeId = null;
-		Faction before = null;
+		String afterId = this.getFactionId();
-		Faction after = this.getFaction();
+		this.updateFactionIndexes(beforeId, afterId);
-		this.updateFactionIndexes(before, after);
 	}
 
 	@Override
 	public void preDetach(String id)
 	{
-		if (!Factions.get().isDatabaseInitialized()) return;
+		String before = this.getFactionId();
-		Faction before = this.getFaction();
+		String after = null;
-		Faction after = null;
 		this.updateFactionIndexes(before, after);
 	}
 
@@ -132,7 +139,7 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 	// There is actually more than one reason we store this data ourselves and don't use the OfflinePlayer#getLastPlayed.
 	// 1. I don't trust that method. It's been very buggy or even completely broken in previous Bukkit versions.
 	// 2. The method depends on the player.dat files being present.
-	//    Server owners clear those files at times, or move their database data around between different servers.
+	// Server owners clear those files at times, or move their database data around between different servers.
 	private long lastActivityMillis = System.currentTimeMillis();
 
 	// This is a foreign key.
@@ -150,7 +157,7 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 	//
 	// Question: Can the title contain chat colors?
 	// Answer: Yes but in such case the policy is that they already must be parsed using Txt.parse.
-	//         If the title contains raw markup, such as "<white>" instead of "§f" it will not be parsed and "<white>" will be displayed.
+	// If the title contains raw markup, such as "<white>" instead of "§f" it will not be parsed and "<white>" will be displayed.
 	//
 	// Null means the player has no title.
 	private String title = null;
@@ -183,6 +190,7 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 	// Null means the player isn't auto claiming.
 	// NOTE: This field will not be saved to the database ever.
 	private transient Faction autoClaimFaction = null;
+
 	public Faction getAutoClaimFaction() { return this.autoClaimFaction; }
 	public void setAutoClaimFaction(Faction autoClaimFaction) { this.autoClaimFaction = autoClaimFaction; }
 
@@ -264,51 +272,34 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 		return !this.getFactionId().equals(Factions.ID_NONE);
 	}
 
-	// This setter is so long because it search for default/null case and takes care of updating the faction member index 
+	// This setter is so long because it search for default/null case and takes
+	// care of updating the faction member index
 	public void setFactionId(String factionId)
 	{
-		// Clean input
+		// Before
-		String target = factionId;
+		String beforeId = this.factionId;
 
-		// Detect Nochange
+		// After
-		if (MUtil.equals(this.factionId, target)) return;
+		String afterId = factionId;
 
-		// Get the raw old value
+		// NoChange
-		String oldFactionId = this.factionId;
+		if (MUtil.equals(beforeId, afterId)) return;
 
 		// Apply
-		this.factionId = target;
+		this.factionId = afterId;
 
 		// Must be attached and initialized
 		if (!this.attached()) return;
 		if (!Factions.get().isDatabaseInitialized()) return;
 
-		if (oldFactionId == null) oldFactionId = MConf.get().defaultPlayerFactionId;
+		if (beforeId == null) beforeId = MConf.get().defaultPlayerFactionId;
 
 		// Update index
-		Faction oldFaction = Faction.get(oldFactionId);
+		Faction before = Faction.get(beforeId);
-		Faction faction = this.getFaction();
+		Faction after = this.getFaction();
 
-		if (oldFaction != null) oldFaction.mplayers.remove(this);
+		if (before != null) before.mplayers.remove(this);
-		if (faction != null) faction.mplayers.add(this);
+		if (after != null) after.mplayers.add(this);
-		
-		/*
-		String oldFactionIdDesc = "NULL";
-		String oldFactionNameDesc = "NULL";
-		if (oldFaction != null)
-		{
-			oldFactionIdDesc = oldFaction.getId();
-			oldFactionNameDesc = oldFaction.getName();
-		}
-		String factionIdDesc = "NULL";
-		String factionNameDesc = "NULL";
-		if (faction != null)
-		{
-			factionIdDesc = faction.getId();
-			factionNameDesc = faction.getName();
-		}
-		Factions.get().log(Txt.parse("<i>setFactionId moved <h>%s <i>aka <h>%s <i>from <h>%s <i>aka <h>%s <i>to <h>%s <i>aka <h>%s<i>.", this.getId(), this.getDisplayName(IdUtil.getConsole()), oldFactionIdDesc, oldFactionNameDesc, factionIdDesc, factionNameDesc));
-		*/
 
 		// Mark as changed
 		this.changed();
@@ -378,8 +369,10 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 			}
 		}
 
-		// NOTE: That we parse the title here is considered part of the 1.8 --> 2.0 migration.
+		// NOTE: That we parse the title here is considered part of the 1.8 -->
-		// This should be removed once the migration phase is considered to be over.
+		// 2.0 migration.
+		// This should be removed once the migration phase is considered to be
+		// over.
 		if (target != null)
 		{
 			target = Txt.parse(target);
@@ -588,7 +581,7 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 
 	public boolean isTerritoryInfoTitles()
 	{
-		if ( ! MixinTitle.get().isAvailable()) return false;
+		if (!MixinTitle.get().isAvailable()) return false;
 		if (this.territoryInfoTitles == null) return MConf.get().territoryInfoTitlesDefault;
 		return this.territoryInfoTitles;
 	}
@@ -661,6 +654,7 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 	{
 		return this.getNameAndTitle(this.getColorTo(faction).toString());
 	}
+
 	public String getNameAndTitle(MPlayer mplayer)
 	{
 		return this.getNameAndTitle(this.getColorTo(mplayer).toString());
@@ -746,7 +740,8 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 	public boolean considerRemovePlayerMillis(boolean async)
 	{
 		// This may or may not be required.
-		// Some users have been reporting a loop issue with the same player detaching over and over again.
+		// Some users have been reporting a loop issue with the same player
+		// detaching over and over again.
 		// Maybe skipping ahead if the player is detached will solve the issue.
 		if (this.detached()) return false;
 
@@ -820,7 +815,7 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 
 			if (MConf.get().logFactionLeave)
 			{
-				Factions.get().log(this.getName()+" left the faction: "+myFaction.getName());
+				Factions.get().log(this.getName() + " left the faction: " + myFaction.getName());
 			}
 		}
 
@@ -830,13 +825,13 @@ public class MPlayer extends SenderEntity<MPlayer> implements EconomyParticipato
 		{
 			EventFactionsDisband eventFactionsDisband = new EventFactionsDisband(this.getSender(), myFaction);
 			eventFactionsDisband.run();
-			if ( ! eventFactionsDisband.isCancelled())
+			if (!eventFactionsDisband.isCancelled())
 			{
 				// Remove this faction
 				this.msg("%s <i>was disbanded since you were the last player.", myFaction.describeTo(this, true));
 				if (MConf.get().logFactionDisband)
 				{
-					Factions.get().log("The faction "+myFaction.getName()+" ("+myFaction.getId()+") was disbanded due to the last player ("+this.getName()+") leaving.");
+					Factions.get().log("The faction " + myFaction.getName() + " (" + myFaction.getId() + ") was disbanded due to the last player (" + this.getName() + ") leaving.");
 				}
 				myFaction.detach();
 			}

src/com/massivecraft/factions/entity/MPlayerColl.java

@@ -43,9 +43,9 @@ public class MPlayerColl extends SenderColl<MPlayer>
 		
 		if (mplayer != null)
 		{
-			Faction before = mplayer.getFaction();
+			String beforeId = mplayer.getFactionId();
-			Faction after = null;
+			String afterId = null;
-			mplayer.updateFactionIndexes(before, after);
+			mplayer.updateFactionIndexes(beforeId, afterId);
 		}
 		
 		return super.removeAtLocalFixed(id);
@@ -63,20 +63,20 @@ public class MPlayerColl extends SenderColl<MPlayer>
 		MPlayer mplayer = null;
 		
 		// Before
-		Faction before = null;
+		String beforeId = null;
 		if (mplayer == null) mplayer = this.id2entity.get(id);
-		if (mplayer != null) before = mplayer.getFaction();
+		if (mplayer != null) beforeId = mplayer.getFactionId();
 		
 		// Super
 		super.loadFromRemoteFixed(id, remoteEntry);
 		
 		// After
-		Faction after = null;
+		String afterId = null;
 		if (mplayer == null) mplayer = this.id2entity.get(id);
-		if (mplayer != null) after = mplayer.getFaction();
+		if (mplayer != null) afterId = mplayer.getFactionId();
 		
 		// Perform
-		if (mplayer != null) mplayer.updateFactionIndexes(before, after);
+		if (mplayer != null) mplayer.updateFactionIndexes(beforeId, afterId);
 	}
 	
 	// -------------------------------------------- //