Duplicate entries in Kibana

Dear all,

I have logstash+elasticsearch+Kibana environment and the problem which bothers me is that I get duplicate messages on Kibana which has a unique filed _id, all the rest are similar.

_id CoWrzOCbT0eiHdSAhtgOsQ

_id uMsM764ZT56xRp8tHn-AJA

How can I get rid of a duplicate ones?

Thanks ahead