“Multiple matches in the address book” means the string that is making it into the SendTo parameter cannot be resolved to a unique mail account (user/group/mail-in db). If you’re using a just a common name or a short name there is more likelihood of a name collision. If so, you should be using a full hierarchical name, internet address or other known-unique addressable form.
To confirm the issue you’ll need to check the value (from where ever it is obtained or by stepping through the code in the debugger) and then check for the duplicate/collision in the $Users view of your Domino Directory.
When I have this issue, I just go to Contacts → Recent Contacts and I delete from there the entries that I need to use, and so far, it works. I hope this works for you