Add array for column value for duplicates [SQL]

Question

I would like to add for duplicate values an array that contains all last_action values. sample_table

client_id  action_timestamp  last_action
201        2020-01-01 09:00  click
201        2020-01-01 09:00  browse
303        2020-01-01 11:00  action

desired_output_table

client_id  action_timestamp  agg_action
201        2020-01-01 09:00  {click,browse}
303        2020-01-01 11:00  {action}