The message could be being re-sent by your Notes client, by your company's server, or by the recipient's company's servers, or by some other server in between.
The first thing you need to do is check whether it could be a problem on your Notes client. In some configurations, Notes uses a file called mail.box as a queue for outbound messages. In other configurations, it mails directly through a queue located on the server. If you are running Lotus Notes 8 or above, you can check the mail.box file by clicking on the File menus, selecting Open, then Lotus Notes Application, and then tabbing down to the File Name field, entering 'mail.box' and clicking the Open button. This should bring you to an 'About' screen with some information about the mail.box file. You can press the Esc key to dismiss this screen, and that should reveal your outbound mail queue. If you see the message in the queue, select it and click Delete Message. If you are on an earlier version of Notes, the procedure will start with clicking the File menu, then selecting Database, then Open.
If the message was not stuck in your outbound mail queue, then this is a matter for your company's Lotus Notes administrators to deal with -- or for the administrators at the recipient's company if the recipient is not in your company. You need to call your IT department and notify them, and the recipient should do likewise.