Outlook XP introduced Autocomplete in the To: field.

This feature shows addresses of people you’ve sent messages to previously as you begin typing in the To: box, and you can select matches . You can’t seed the list with names from your address book, the feature collects them when you send new messages or reply.

Needless to say its blessing is also its curse, as the list can contain mulitple identical names and old and unusable addresses

You can clear the autocomplete cache by:

deleting the *.NK2 file for your profile

delete one address at a time by using the down arrow key to select it as you type and then hit [Delete].

Hate it? You can turn it on or off in Tools, Options, Email Options, Advanced Options, Suggest names.