mergebug
#### My code

```python
def mer_new():
    data1 = read_file.reduce_mem_usage(pd.read_csv('E:\\skysun\\JDA\\jdata\\jdata_action.csv', sep=',', nrows=6000000,
                                                   dtype={'acoustic_data': np.int16, 'time_to_failure': np.float64}))
    data1 = data1[['user_id', 'sku_id', 'action_time', 'type']]
    print('行为数据:%d' % len(data1))
    print('\n')
    data2 = read_file.reduce_mem_usage(pd.read_csv('E:\\skysun\\JDA\\jdata\\jdata_comment.csv', sep=',', nrows=6000000,
                                                   dtype={'acoustic_data': np.int16, 'time_to_failure': np.float64}))
    data2 = data2[['sku_id', 'good_comments', 'bad_comments']]
    print('评论数据:%d' % len(data2))
    print('\n')
    data = pd.merge(data1, data2, how='left', on='sku_id')
    print('合并数据：%d'%len(data))
    # data =pd.concat([data1,data2],axis=1,join='outer')
    data = data.drop_duplicates()
    data.to_csv('data/action_comment.csv', mode='w', index=False)
mer_new()
```
#### Problem description

when I merge two table,I set the 'how='left'', so the length of original data should be equal the length of merge data.(the second table's length is 1774233)

But it return


len(data1):6000000
len(data)：284428008
It does not equal!!!! Why???







