Outlook - strange behavior

At my work, I use a computer with Windows 2000 and a version of Outlook close to 2000 (don't have all the details here at home).

Something odd has happened three times now. Twice today and once about 3-4 months ago.

In both instances I received an e-mail from a customer (different customers all three times) and responded to their question, request, etc. Then, I get a phone call, e-mail, or someone walking in my office (we have about 12 employees) telling me they got the e-mail reply I just sent. A few months ago it was just one coworker in my office (that I know of... my customers may write it off as an error and not tell me about it). Today, it was two coworkers that I know of and two customers belonging to two separate companies (competitors).

The original e-mails in all instances were to me ONLY and were not BCC/CC to anyone else. I know this because many of my customers are competitors with each other and would have no reason to BCC/CC each other on these e-mails. I also know I am not clicking "Reply All".

I did some basic research and could not find this using Google, but maybe I'm not searching properly.

Have any of you heard of anything like this happening? Any thoughts? I don't have much detail on the system but the problem seems so strange I thought I'd bring it up.

Thanks,

BR