我认为您boolean不是string，所以需要删除'：
df = pd.DataFrame({'Column_A': ['AAA','AAA','ABC','CDE'],
'checked': ['0','0','1','0'],
'duplicate': [True, True, False, False]})
df['flag'] = np.where((df['checked'] == 'Y') &(df['duplicate'] == True), 'Y', '0')
print (df)
Column_A checked duplicate flag
0 AAA 0 True 0
1 AAA 0 True 0
2 ABC 1 False 0
3 CDE 0 False 0
或者，如果与boolean列比较，== True则可以省略：
df['flag'] = np.where((df['checked'] == 'Y') &(df['duplicate']), 'Y', '0')
print (df)
Column_A checked duplicate flag
0 AAA 0 True 0
1 AAA 0 True 0
2 ABC 1 False 0
3 CDE 0 False 0
另外如果需要检查checked需要，'因为strings：
df['flag'] = np.where((df['checked'] == '0') &(df['duplicate'] == True), 'Y', '0')
print (df)
Column_A checked duplicate flag
0 AAA 0 True Y
1 AAA 0 True Y
2 ABC 1 False 0
3 CDE 0 False 0
编辑：
df['flag'] = '0'
mask = (df['checked'] == '0') &(df['duplicate'])
df.loc[mask, 'flag'] = 'Y'
print (df)
Column_A checked duplicate flag
0 AAA 0 True Y
1 AAA 0 True Y
2 ABC 1 False 0
3 CDE 0 False 0
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