unable to purchase in app purchase

I'm trying an in app purchase but it says, "unable to purchase This item is for an app that was purchased by a different Apple ID. To buy this item with this Apple ID, you must first purchase the app."

The app was purchased 6 yrs ago.

First, I have no idea which other ID I would have purchased the app from, unless I was signed in as @me.com instead of @ mac.com, which apple store employees have told me should be interchangeable?

But even if this app was originally purchased with a different ID how has this app made it from phone to phone (I get a new phone every 2 yrs) if it was on an old ID? When Apple loaded up my new phones each time, why would an app purchased on a different device under a different ID get loaded on?

Also, how can I repurchase the original app with my "new" ID? When I go to the app store it says, "open." There is no price listed or option for

Thanks for any help on this.