Pandas Dataframe: how to add column with number of occurrences in other column

Question

I have to following df:

Col1    Col2
test    Something
test2   Something
test3   Something
test    Something
test2   Something
test5   Something

Answer 1

I can't get the other answers to work when I want to retain more columns than just the two columns Col1 and Col2. Below works well for me with any number of other columns retained.

df['Occur'] = df['Col1'].apply(lambda x: (df['Col1'] == x).sum())