Calender ghosting on my old phone

[Doily]

Hi, All the calendar entries that I make on my 2017 A5 also appear on S Planner on my old S4 Mini that I gave to my dad. He also gets all my reminders on his phone. I also get them on my phone. The entries he makes on the S4 Mini do not show up on my phone.
How can I stop this duplication please (neither of us want to stop using our calendars). Thanks very much.

 

[B. Diddy]

Welcome to Android Central! Did you make sure to completely remove your Google account from the S4 mini before setting it up for him? Ideally, you should first remove your Google account (in Settings>Accounts), then do a factory reset, and then set the phone up with your father's Google account.