There are two reasons why you can’t believe slander/lashon hara even if you hear two or more people saying the same thing.
1. Is that if it is said for no good reason then the speakers are bad people and how can you believe what a bad person said verses the person being spoken about who is assumed to be a kosher person.
2. Is that even if what they are saying is to warn others of impending danger that doesn’t give anyone the right to believe what they are saying as truth, only two witnesses in court who are official witnesses are believed unequivocally.
Any other time one can and should only be cautious take the proper precautions to protect yourself and research and investigate the matter to see if it is really true or not.
The Gemara in Pesachim 113b says that a rebbe if he feels he could trust his student as if he is two in order to stay away from someone who is doing inappropriate things according to the student. The Chofetz Chaim explains that this is a special circumstance where the relationship between rabbi and student is so close and trustworthy that this student is like two witnesses testifying in court and is therefore unequivocally believed but in general two people or more are not believed for what they say if it is lashon hara since slander usually leads to, comes with lies or at least leaving out information or exaggerating so in normal circumstances it cannot be believed no matter how many people say it it can only be taken into consideration as a concern, investigated and take proper precautions.