Docs are incpmplete for concat
There is an issue with concat:
I expect it not to  add rows from a second df in case there are such indexes in a first one.
But this doesn't work for me. After a lot of search I found that this is because I have duplicate values in indexes in both df.
It would be great to warn about such a behavior in standard function documentation.

That is how I found that it is not a bug, but rather a feature:
https://stackoverflow.com/questions/24089922/merge-multiple-dataframes-with-non-unique-indices/24091063#24091063

Frankly speaking, I would prefer if there was an option to leave only "unique" indexes and in case they are present in first df - skip them all together during the concat