yep

Shogi.Domain/Aggregates/Session.cs

@@ -4,28 +4,29 @@ namespace Shogi.Domain;
 
 public class Session
 {
-	public Session(
-		string name,
-		string player1Name)
-	{
-		Name = name;
-		Player1 = player1Name;
-		Board = new(BoardState.StandardStarting);
-	}
+  public Session(
+    string name,
+    string player1Name)
+  {
+    Name = name;
+    Player1 = player1Name;
+    Board = new(BoardState.StandardStarting);
+  }
 
-	public string Name { get; }
-	public ShogiBoard Board { get; }
-	public string Player1 { get; }
-	public string? Player2 { get; private set; }
+  public string Name { get; }
+  public ShogiBoard Board { get; }
+  public string Player1 { get; }
+  public string? Player2 { get; private set; }
 
-	public void AddPlayer2(string player2Name)
-	{
-		if (Player2 != null) throw new InvalidOperationException("Player 2 already exists while trying to add a second player.");
-		Player2 = player2Name;
-	}
+  public void AddPlayer2(string player2Name)
+  {
+    if (Player2 != null) throw new InvalidOperationException("Player 2 already exists while trying to add a second player.");
+    if (Player1.Equals(player2Name, StringComparison.OrdinalIgnoreCase)) throw new InvalidOperationException("Player 2 must be different from Player 1");
+    Player2 = player2Name;
+  }
 
-	public bool IsSeated(string playerName)
-	{
-		return Player1 == playerName || Player2 == playerName;
-	}
+  public bool IsSeated(string playerName)
+  {
+    return Player1 == playerName || Player2 == playerName;
+  }
 }