test blend of OP question query and my answer, Sql Server

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CREATE TABLE dbo.VezaUgovorPartner (
    PartnerId INT NOT NULL,
    UgovorId INT NOT NULL)

CREATE TABLE dbo.Partner (
    PartnerId INT NOT NULL,
    Naziv VARCHAR(100) NOT NULL)

INSERT INTO VezaUgovorPartner
VALUES (1,'1265'), (2,'1265'), (3,'1265'), (4,'1264'), (5,'1263'), (6,'1262'),
         (7,'1261'), (8,'1260'), (9,'1259'), (10,'1258'), (11,'1257'), (12,'1256')
         
INSERT INTO Partner
VALUES (1,'MARK'),
(2,'LISA'),
(3,'MAY'),
(4,'LINDA'),  
(5,'MARTINA'),
(6,'MARKO'),                                                                                                                                                                                
(7,'VIDAKS'),
(8,'PEEKS'), 
(9,'MARCUS'),
(10,'MARKO'),                                                                                                                                                                                
(11,'MATIA'),                                                                                                                                                                                 
(12,'POPOVIC')
         
         
SELECT vup.UgovorId as ID, 
     STUFF((SELECT DISTINCT ', ' + p2.Naziv  
           FROM VezaUgovorPartner vup2 
           INNER JOIN [Partner] p2 ON vup2.PartnerId = p2.PartnerID 
           WHERE vup2.UgovorId = vup.UgovorId   
           FOR XML PATH('')), 1, 2, '') NUMBER
FROM VezaUgovorPartner vup 
INNER JOIN [Partner] p on vup.PartnerId = p.PartnerID 
GROUP BY vup.UgovorId
ORDER BY vup.UgovorId DESC

View schema
Execution time: 0,03 sec, rows selected: 10, rows affected: 24, absolute service time: 0,17 sec