Back in the old days many people would configure their loopbacks as /30s because we were told that /31 and /32 were not valid subnet masks. It is true that OSPF should have cured this (a loopback interface is always advertised as a /32 unless you change the OSPF network type. BUT one day I happen to see
Jul 23 11:46:53 CDT: %OSPF-4-CONFLICTING_LSAID: Process 100 area dummy area: LSA
origination prevented by LSA with same LSID but a different mask
Existing Type 5 LSA: LSID 10.120.0.216/30
New Destination: 10.120.0.216/32
Turns out someone had configured a loopback address of 10.120.0.218/30 and in a different box 10.120.0.216/32 creating the situation above. I found the boxes by doing sho ip bgp <prefix> <mask> then check the AS number table. Now have to just change the subnet mask of the 10.120.0.218 interface and all will be well
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