hi,
You can do this using 2 ways

Using Workflow and using Trigger.

Approach 1:
Create a text field in Contact object as Account Name Unique
Create a Work Flow on Contact and set the criteria contact:Account Name Is Not Null
Create a Workflow Actions as Field Update and Select the Account Name Unique Field on contact object and Set the value as AccountId on Contact and click on done.
Activate the Work flow.

Approach 2:
create a master detail relationship and than on master create a roll up summary field of child with count. 
then you write validation on rule rollup summary field to check for >1.
now it will give you error if it has more than one record for same master detail relation values..

trigger oneToOne on Opportunity (before insert, before update) {
    Set<Id> accids = new Set<Id>();
    Map<Id, Account> mapAccount = new Map<Id, Account>();
    
    for(Opportunity e : trigger.New) {
        accids.add(e.Account);
    }
    
    List<Account> listInterest = [SELECT Id, Opp_count__c FROM Account WHERE Id =: accids];
    
    for(Account intr : listInterest) {
        mapAccount.put(intr.Id, intr);
    }
    
    for(Opportunity e : trigger.New) {        
        if(mapAccount.get(e.Account).Opp_count__c == 1) {
            e.addError('Already an employee has been associated with this interest');
        }
    }
}