Python 当密钥是复合且唯一时，Pandas合并抱怨标签不唯一

我正在try 合并两个收件箱，以便最终得到一个列数相同但行计数增加的收件箱.

import pandas as pd, numpy as np

data1 = [['date'  ,    'symbol', 'value'],
         ['1999-01-10', 'AAA',      101],
         ['1999-01-11', 'AAA',      201]]
I am trying to merge two dataframes such that i end up with one with same number of columns but row count should increase


import pandas as pd, numpy as np

data1 = [['date'  ,    'symbol', 'value'],
         ['1999-01-10', 'AAA',      101],
         ['1999-01-11', 'AAA',      201]]

data2 = [['date'  ,    'symbol', 'value'],
         ['1999-01-10', 'BBB',      101],
         ['1999-01-11', 'BBB',      201]]


df1 = pd.DataFrame(data1[1:], columns=data1[:1])
df2 = pd.DataFrame(data2[1:], columns=data2[:1])
df = df1.merge(df2, on = ['date', 'symbol'], how='outer')

上面的代码在合并行上产生错误:

ValueError: The column label 'date' is not unique.
For a multi-index, the label must be a tuple with elements corresponding to each level.

我知道在上面的情况下我可以通过pd.CONCAT实现我想要的目标，但我想理解为什么合并在这里失败，因为日期+符号的复合键是不同的/唯一的？ 而且我不明白关于多索引的部分. 除了这些 pyramid 上的"自然"指数之外，没有任何指数.