Why are my contacts being deleted?

Cliffs:
My Lumia 925 broke
Dad has two Lumia 920's(original/new)
Dad has not used old Lumia in about a year
Dad gives me old Lumia 920
I transfer My 925 SIM Card in to his old 920
His Original Microsoft email account is linked on both phones
My contacts show up on 920 as well as his
I now receive his Email/ can't unlink his Email
I delete his contacts on the old 920(Now my phone)
Contacts somehow get deleted on his new Lumia 920
Dad has my contacts on his new 920??? Wth
How can I stop this
Sorry my first post, I'm confused as hell. IDK where to put this