finding duplicate numbers in a select distinct statement

Hello all,

I am trying to do the following query but I would like to know how
many rows were deduplicated in the process:

psql -d foo -c 'SELECT DISTINCT ON (error) error,ev_text FROM
clients_event_log' -o fullfoo

For example, I would want the output to be the following where count
is the number of error rows that were deduplicated.

count | error | ev_text
55 | 525152 | Some text
72 | 125124 | Some other text

I imagine it's using the count function somewhere, just not sure how
to embed that.