Leetcode: Valid Sudoku @Python

class Solution:
    # @param board, a 9x9 2D array
    # @return a boolean
    def isValidSudoku(self, board):
        for i in range(9):
            dup=[]
            for j in range(9):
                if board[i][j]!='.':
                    if board[i][j] in dup:
                        return False
                    else:
                        dup.append(board[i][j])
        for j in range(9):
            dup=[]
            for i in range(9):
                if board[i][j]!='.':
                    if board[i][j] in dup:
                        return False
                    else:
                        dup.append(board[i][j])
        for i in [0,3,6]:
            for j in [0,3,6]:
                dup=[]
                for m in range(3):
                    for n in range(3):
                        if board[i+n][j+m]!='.':
                            if board[i+n][j+m] in dup:
                                return False
                            else:
                                dup.append(board[i+n][j+m])
        return True