How to join columns sharing the same name within a dataframe

Question

I'm new to pandas. My df looks like this:

  A   A   A   B   B   B
a NaN NaN 2   NaN NaN 5
b NaN 1   NaN 9   NaN NaN
c 3   NaN     NaN 7   NaN

How can I get

  A   B 
a 2   5
b 1   9 
c 3   7

It looks like merge, join are for more than one dataframe. I have also tried

df.groupby(by=[A,B], axis=1)

but got

ValueError: Grouper and axis must be same length