BUG: mangle_dup_cols in read_csv replacing existing cols when conflict with target col
- [x] closes #14704
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry


This ensures that already existing column names are kept. The index of the duplicated col is one higher in this case
@gfyoung if any comments Thx I think its worth making this a sub-section in the whatsnew docs to highlite it. Added the sub section @jreback greenish thanks @phofl  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  Are you sure? previous commit was green, only merged master and modified whatsnew afterwards. not sure at all - if it's not a problem on master then ignore 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