BUG: mangle_dup_cols in read_csv replacing existing cols when conflict with target col

[phofl]

• closes read_csv(...,mangle_dupe_cols=True) causes silent data loss for certain column names. Request introduction of mangle_dupe_cols_str #14704
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

This ensures that already existing column names are kept. The index of the duplicated col is one higher in this case

[jreback]

@gfyoung if any comments

[gfyoung]

Nice work!

The modified expected results for the tests will take a little getting used to, but they of course make total sense.

[phofl]

Thx

[jreback]

I think its worth making this a sub-section in the whatsnew docs to highlite it.

[phofl]

Added the sub section

[phofl]

@jreback greenish

[jreback]

thanks @phofl

[jreback]

https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=69777&view=logs&j=cb04ab33-336b-54ee-68fe-dcd3abde835b&t=0558d00d-ff4a-5538-0de9-a0e7f449700f

hmm this actually be related

[phofl]

Are you sure? previous commit was green, only merged master and modified whatsnew afterwards.

[jreback]

not sure at all - if it's not a problem on master then ignore

[phofl]

Have seen similar things in the past like https://github.com/pandas-dev/pandas/runs/4331169003, hence assumed this would be unrelated. Lets see if it passes on master