BUG: pd.wide_to_long can't handle stubname equal to suffix

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Reproducible Example

import pandas as pd
import numpy as np
np.random.seed(123)
df = pd.DataFrame({"year1" : {0 : 4.5, 1 : 1.7, 2 : .8},
                   "year2" : {0 : 2.5, 1 : 1.2, 2 : .7},
                   "year3" : {0 : 2.6, 1 : 1.6, 2 : .6},
                   "year4" : {0 : 3.2, 1 : 1.3, 2 : .1},
                   "X"     : dict(zip(range(3), np.random.randn(3)))
                  })
df["id"] = df.index
pd.wide_to_long(df, stubnames="year", i="id", j="year")  # AttributeError: 'DataFrame' object has no attribute 'str'

Issue Description

pd.wide_to_long raises an AttributeError when the parameters stubnames and j (suffix) are equal. The AttributeError does not really tell you what's the problem.

Expected Behavior

I see two potential fixes:

• Raise an AttributeError that tells you that stubnames and j can't be identical
• Make it so that stubnames and j can be identical

Installed Versions

commit : 4bfe3d07b4858144c219b9346329027024102ab6 python : 3.9.5.final.0 python-bits : 64 OS : Windows OS-release : 10 Version : 10.0.19044 machine : AMD64 processor : Intel64 Family 6 Model 78 Stepping 3, GenuineIntel byteorder : little LC_ALL : None LANG : None LOCALE : English_United Kingdom.1252 pandas : 1.4.2 numpy : 1.21.2 pytz : 2021.3 dateutil : 2.8.2 pip : 21.1.2 setuptools : 57.0.0 Cython : None pytest : None hypothesis : None sphinx : None blosc : None feather : None xlsxwriter : None lxml.etree : None html5lib : None pymysql : None psycopg2 : None jinja2 : None IPython : None pandas_datareader: None bs4 : None bottleneck : None brotli : None fastparquet : None fsspec : None gcsfs : None markupsafe : None matplotlib : 3.4.3 numba : None numexpr : None odfpy : None openpyxl : 3.0.9 pandas_gbq : None pyarrow : None pyreadstat : None pyxlsb : None s3fs : None scipy : 1.7.1 snappy : None sqlalchemy : None tables : None tabulate : None xarray : None xlrd : None xlwt : None zstandard : None