Bug with Apple ID - same email address but two accounts?

I have been having issue with my Apple ID.

So I have been trying to sign in to iMessage on my Mac but it kept saying there is activation error.

Which led me to realising that although my iPhone is connected to the same email address and also the number, they are apparently not linked to my Apple ID which is strange because my Apple ID is of the same email.

I then tried to add my mobile number to my Apple ID (settings on iPhone), it would say the mobile is linked to another account.

Then when I tried to add my email address to my iMessage (settings on iPhone), it would not allow me / the button is unclickable.

I have updated both Mac OSX and iOS to the latest and also signed in and out of the Apple ID multiple times. I wanted to contact Apple Support but it is "limited" lol.

What can be done?

Any help please?