BUG: Fix DataFrame.astype(ExtensionDtype) with duplicate column names

[jschendel]

• xref Fatal error with astype if duplicate columns are supplied for categorical #24704 (comment) (this fixes item 1)
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #24717 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #24717   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52321    52321           
=======================================
  Hits        48337    48337           
  Misses       3984     3984

Flag	Coverage Δ
#multiple	90.8% <100%> (ø)	⬆️
#single	43.06% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/generic.py	96.62% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #24717 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #24717   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52321    52321           
=======================================
  Hits        48337    48337           
  Misses       3984     3984

Flag	Coverage Δ
#multiple	90.8% <100%> (ø)	⬆️
#single	43.06% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/generic.py	96.62% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d46d2a5...84e6033. Read the comment docs.

[jreback]

thanks @jschendel