BGP uses two mechanism to prevent loops:-

When a router learns routes from an iBGP peer, that router does not advertise the same routes to another iBGP peer.
By using AS_PATH - When advertising to an eBGP peer, a BGP router adds its own ASN to the AS_PATH. If a BGP router receives an update and the route advertisement lists an AS_PATH with its own ASN, the router ignores that route.

Note - A BGP router does not add its ASN when advertising to an iBGP peer.   _{Your Comment}