Which device is master

Hi

i have an iPhone and laptop which are synced on iCloud in terms of contacts and calendar with a lot of information inputted over a few years.

For for a reason I can't remember, on my desktop iMac I have seperate contacts and calendar info.

I now want my desktop to be part of the iCloud sync but I'm afraid of the consequences if I do. Will the phone and laptop info be replaced by the desktop's info or vice versa? Or will it amalgamate? If it does amalgamate and I have duplicate info, will I suddenly have duplicates?

thanks in anticipation