SourceSleuth is an addition to SourceBans, originally created in 2012, that looks for potential duplicated accounts by comparing IP addresses. A ban caused by a duplicated IP is rare, which allows SCG admins to investigate each occurance to assure it is a deserved ban rather than a fluke, as IP addresses can change very easily.
StAC or STeph's AntiCheat, is an anticheat SourceMod plugin created by Sapphonie. It uses packet detection and aim angle viewing to detect cheaters, along with other methods to look for other oddities. SCG uses it on assorted TF2 servers, as well as a custom modified version in the Open Fortress server.