Drop a temporary table if it exists

From SQL Server 2016 you can just use

 DROP TABLE IF EXISTS ##CLIENTS_KEYWORD

On previous versions you can use

IF OBJECT_ID('tempdb..##CLIENTS_KEYWORD', 'U') IS NOT NULL
/*Then it exists*/
DROP TABLE ##CLIENTS_KEYWORD
CREATE TABLE ##CLIENTS_KEYWORD
(
   client_id INT
)

You could also consider truncating the table instead rather than dropping and recreating.

IF OBJECT_ID('tempdb..##CLIENTS_KEYWORD', 'U') IS NOT NULL
  TRUNCATE TABLE ##CLIENTS_KEYWORD
ELSE
  CREATE TABLE ##CLIENTS_KEYWORD
  (
     client_id INT
  )