I had a requirement to restrict multiple approval for a single user to a single record. There were multiple steps of approval process and the approver may be in different queue or as a manager for a user.

So, I implemented it using a trigger. In this article I’ll demonstrate how to implement it.
Here Application__c is the object, on which 3 steps approval process has implemented.
A custom check box field Check_Approver__c has set value to true, in all 3 steps field update action.

In below trigger I filtered the ProcessInstanceStep object records with current user Id in ActorId and the object Id (here object Id means record Id) in ProcessInstance.TargetObjectId.

• If an User will approve or reject a record, then the SOQL query will return a single record.
• If the SOQL query return a record, then trigger will throw an error message.

Trigger source code:

trigger ApplicationTrigger on Application__c (before Update)
{
    if(trigger.isUpdate){

        Id currentUserId = UserInfo.getUserId();
        for(Application__c sf: trigger.new){   
                  
            if(sf.Check_Approver__c == true){
              
                List<processinstancestep> existingApprovals = [SELECT ActorId
                                    FROM ProcessInstanceStep WHERE ProcessInstance.TargetObjectId = :sf.Id
                                    AND (StepStatus = 'Approved' OR StepStatus = 'Rejected')
                                    AND ActorId = :currentUserId];
                                      
                if(existingApprovals != null){
                  
                    if(existingApprovals.size() > 0){

                        sf.addError('You have already approved or rejeted the record.');
                    }
                }                  
             }
         }                         
    }
}