how to remove dublicate in sqlit database

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in the following code i want to use the rollno is the primary key and when the dublicate rollno is entered it should to show the message that "this number is already exist"

i have used the following primary key but it doesnot work

public class DataHaandler {
	
	public static final String NAME="name";
	public static final String EMAIL="email";
	public static final String TABLE_NAME="mytable";
	public static final String DATA_BASE_NAME="mydatabase";
	public static final int DATABASE_VERSION=1;
	public static final String TABLE_CREATE="create table"+ TABLE_NAME+"("+NAME + "INTEGER PRIMARY KEY AUTOINCREMENT,"+TABLE_NAME+" TEXT,"+EMAIL+");";
	DataBaseHelper dbhelper;
	Context ctx;
	SQLiteDatabase db;
	public  DataHaandler(Context ctx)
	{
		this.ctx=ctx;
		dbhelper=new DataBaseHelper(ctx);
	}
	private static class DataBaseHelper extends SQLiteOpenHelper
	{
		public DataBaseHelper(Context ctx)
		{
		super(ctx,DATA_BASE_NAME,null,DATABASE_VERSION);	
		}

		public DataBaseHelper(Context context, String name,
				CursorFactory factory, int version) {
			super(context, name, factory, version);
			// TODO Auto-generated constructor stub
		}

		@Override
		public void onCreate(SQLiteDatabase db) {
			// TODO Auto-generated method stub
		try{
			db.execSQL(TABLE_CREATE);
		}
		catch(SQLException e)
		{
			e.printStackTrace();
		}
		}
		
		@Override
		public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
			// TODO Auto-generated method stub
			
		db.execSQL("DROP TABLE IF EXISTS mytable");
		onCreate(db);
			
		}
		
	}
	public DataHaandler open()
	{
		db=dbhelper.getWritableDatabase();
		return this;
	}
	
	public void close()
	{
		dbhelper.close();
	}
	
	public long insertData(String name,String email)
	{
		ContentValues content=new ContentValues();
		content.put(name, name);
		content.put(email, email);
		return db.insert(TABLE_NAME, null, content); 
	}
	
	public Cursor returnData()
	{
		return db.query(TABLE_NAME, new String[]{NAME,EMAIL}, null, null,null, null, null);
		
	}


}