Managing personal Outlook.com account settings, security, and privacy
The system will simply not allow 2 people to have/use the same email address. I supect either they have mis typed their email address or are spammer who often "spoof" (fake) the email address.
1. Some one may have "spoofed" your email address.
Please read more about it below.
**What Can You Do if Someone Has Spoofed Your Email Address?**
Why am I getting spam from myself?