How to SUM values with same value by using WHERE IN()
The IN predicate will either evaluate to TRUE or FALSE (or NULL) exactly once for each row, so you won't get any multiplier effect from that. You can use a Common Table Expression (CTE, only supported in 8+) or a temporary table
CREATE TABLE vo_plurep
( ID_PLUREP int not null
, PRE_PRP int not null
);

insert into vo_plurep (id_plurep, pre_prp) values (1,1),(2,2);

CREATE TEMPORARY TABLE T (id_plurep int not null);

insert into T (id_plurep) values (1),(1);

SELECT SUM(PRE_PRP) AS IMPORTO
FROM vo_plurep
JOIN T
    ON T.id_plurep = vo_plurep.id_plurep


IMPORTO
2

Fiddle