It's a messy situation, as some workloads (*cough* Teams *cough*) don't check for the presence of an existing/matching object, thus allowing you to have duplicates, while Exchange in general will throw an error. Now, if the Guest user has redeemed the invitation, generally speaking you should be able to use this object instead of a contact, as it has all the required mailflow-related attributes.
If you insist on having a contact object, you can always remove the Guest user. Or, you can simply unhide him from the GAL and use the corresponding entry.
