No error message in discovery rule when adding the same checks in 6.0 and 6.4 (master - ok)
Steps to reproduce:

1. Add network discovery
2. Add "ICMP ping" check
3. Add one more "ICMP ping" check

Result: no error when adding the same check. The error only appears when saving the discovery rule, but this is wrong. Works as expected with other duplicate checks, for example when add FTP

Expected result: "Check already exists" right after adding the second duplicate check