Emails are not sent to your computer directly from a person's PC. If they were, then I would answer this question "they're using a VPN".
But since emails are routed through an email server, then this makes a lot of sense.
The person sending email to you sends their message, which is then picked up by their email provider that has an address of 123.456.789.100 and sends it along to you. Another person sends an email who is using the same provider, therefore the same IP address for the server.
You're seeing the email server's IP. Online email service providers don't let you see the origin IP address of the machine the email was sent from, unless you're a powerful government organization with handy pieces of unlawful legislation...
Your question is not exactly clear. While an IP address is unique and has a single owner, that owner can assign that address, to say different customers. What problem do you face? – Ramhound – 2013-10-07T17:14:57.043
IP Addresses can be shared and ARE shared all over the place. Everyone in your house (most likely) shares an IP Address, if that wasn't enough your computer (most likely) is not the IP Address that you'll see but your mail servers. If I'm using gmail, you're using gmail, and some other person is using gmail, it could be possible that all of our email's have the same source IP Address (from the destination's perspective) because they come from the same mail server. – u8sand – 2013-10-07T17:17:58.343
maybe you are reading yahoo's ip address. you could paste the header here and replace sensitive info with equivalent info. – barlop – 2013-10-07T17:30:40.617