What is the tcode to compare 2 roles in order to find out the duplicate transaction code?

For role comparison both the roles must be in the same system, in same client. 
    
Transaction code SUIM -> Comparison-> Roles 
    
If the roles are in different system, then transport the role into one of the system and do comparison.  If no transport connection defined then, you can use the upload and download option in the PFCG.  

Steps for Role Comparing: 

1. Run the t-code SUIM 
    
2. Go To Comparison and select the option of roles 
    
3. Click on Across systems option it will give option to select the sys name under Remote Comparison there enter the SYS ID between which system you want to do comparison and put the role name in compare role section then execute it will give you the result. 
    
4. If there are any difference between then the  t-codes it will be in red color otherwise in yellow?
 

How do I use SUIM to compare Roles which are different Clients of the same system?

The RFC destinations are client independent but the login details provided in the RFC destination are client dependent right?

So all you need is to define to RFC destinations (looping) pointing to the same system.

In one of the RFC destinations, you can provide the user ID details of the source system client while in the other, you can provide the details of the target client.

So now all you have to do is goto SUIM --> Comparisons--> Cross systems and provide the RFC destination details and the role names.

or

Go to PFCG --> In menu click on utilities --> Click on role comparison tool or use t-code PFCG_CMP

Give the role name and select the comparison destination give destination and execute.

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