CategoricalIndex.reindex raises on duplicate indexer

[adbull]

Code Sample, a copy-pastable example if possible

import pandas as pd

# works fine
x = pd.Series([0])
x.reindex([0,0])

# ValueError: cannot reindex with a non-unique indexer
x.index = x.index.astype('category')
x.reindex([0,0])

Problem description

When a series or frame has a categorical index, it cannot be reindexed using an indexer that has duplicate values. This is inconsistent with other indexes, and makes categorical indexes less useful.

Expected Output

No error.

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.15-200.fc28.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.24.1
pytest: 4.3.0
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.5
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 4.2.1
sphinx: None
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: 2.6.2
feather: None
matplotlib: 3.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.3.1
bs4: 4.7.1
html5lib: None
sqlalchemy: 1.2.18
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

[gfyoung]

cc @jreback @toobaz

[batterseapower]

This was broken by #23963 which was trying to fix #21809. The problem is that this line should be checking self.is_unique instead of target.is_unique:

pandas/pandas/core/indexes/category.py

Line 602 in bdcab11

if not target.is_unique: