Incorrect delivery of a message is extremely unlikely. There are two reasons you might receive messages not specifically addressed to you:

• The sender thinks you're someone else. It's not possible for two people to share the same Yahoo! ID/email address, so the sender might have made a small typo and caused the message to be sent to the wrong account.
• If you can't find your address in the "To:" or "Cc:" field, it might be in the "Bcc:" ("blind carbon copy") field of the message. It will look as if you've received someone else's message since your address won't appear anywhere in the header.