How to stop my iPhone from merging contacts?
It appears to be a feature of Office 365. Similar contacts get ‘linked’ automatically. Unlinking is possible, although, when I attempted it myself, one of the linked contacts was lost completely. There is no way of disabling the ‘feature’ at the moment.

Unlinking contacts may be achieved as follows:


Open Outlook 2013 or newer and switch to People;
Ensure that your Current View is set to People;
Choose any of the contacts that you believe to be linked;
Press ‘Link Contacts…’ under View Source at the right;
Press × for each contact listed as they will reappear in your list of contacts.