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  • Eiji Tanioka thinks this is interesting:

no-prepend replace-as dual-as optional keywords allows the remote peer to user either ASN for the BGP session. The remote BGP router will peer with the ASN in the router process statement or the alternate ASN in the local-as configuration.

From

Cover of IP Routing on Cisco IOS, IOS XE, and IOS XR: An Essential Guide to Understanding and Implementing IP Routing Protocols

Note

dual-as keyword can peering router to use Alternate or Real ASN to peer me.