CREATE UNIQUE INDEX IF NOT EXISTS in postgreSQL

You can check, if an index with a given name exists with this statement.

If your index name is some_table_some_field_idx

SELECT count(*) > 0
FROM pg_class c
WHERE c.relname="some_table_some_field_idx" 
AND c.relkind = 'i';

Starting from Postgres 9.5 you can even use

CREATE INDEX IF NOT EXISTS