Hello shayforeal,
Thank you for using the Apple Support Communities! Based on the details you've provided, your contacts are appearing on a separate iPhone other than yours. I am happy to help.
You are right in bringing up the Apple ID account. Typically, when two iPhones are caught sharing each others contacts, messages, etc. it is because they are signed in with the same Apple ID in iCloud settings. It sounds like you've checked on both iPhones under Settings > tap your name at the top > iCloud, to make sure you are both signed in to separate accounts. Is that confirmed?
If you are definitely using separate iCloud accounts, then we need to explore other possibilities. Contacts sync via accounts, in general. Can you compare the type of accounts you have set up on each iPhone to look for similarities. This can be email accounts, social media accounts, etc. I would also like you to navigate to Settings > Contacts, on each device, and see if you are using the same Default account in that setting. Manage and delete contacts on your iPhone, iPad, or iPod touch - Apple Support
Reach back out with your responses and what you find.
Regards.