How to sort by 2 columns and keep the top row from each group created?

Question

I have two csv file of the following format:

column1,column2,user,column4,column5,column...column14  

I would like to sort the files based on user as first key and then based on column14 in descending order which is a number representing a timestamp.

The result would be like:

file1:

A,AA,jim,XX,YY,....,1485771395    
A,AA,jim,XX,YY,....,1485771395    

etc

I would like then from these files to keep only the first row of each cluster i.e. only

A,AA,jim,XX,YY,....,1485771395  

How can I do that?

Update:
Example input:

"1/30/2017 11:14:55 AM",Valid customer,jim.smith,NY,1485771295      
"1/26/2017 5:06:11 AM",New customer,john.doe,CA,1485403571  
"1/26/2017 8:02:01 PM",Valid customer,jim.smith,NY,1485457321  
"1/30/2017 4:14:30 AM",New customer,tim.jones,CO,1485746070  
"1/30/2017 11:09:36 AM",New customer,tim.jones,CO,1485770976  
"1/30/2017 11:14:50 AM",Valid customer,jim.smith,NY,1485771290      
"1/22/2017 11:51:51 AM",New customer,tim.jones,CO,1485082311  

The last column is the epoch of the first column to be able to sort the rows using a number instead of a string.
So the expected output would be:
a) Sorting on name and on timestamp in reverse order so we get:

"1/30/2017 11:14:55 AM",Valid customer,jim.smith,NY,1485771295    
"1/30/2017 11:14:50 AM",Valid customer,jim.smith,NY,1485771290        
"1/26/2017 8:02:01 PM",Valid customer,jim.smith,NY,1485457321  
"1/26/2017 5:06:11 AM",New customer,john.doe,CA,1485403571  
"1/30/2017 11:09:36 AM",New customer,tim.jones,CO,1485770976  
"1/30/2017 4:14:30 AM",New customer,tim.jones,CO,1485746070  
"1/22/2017 11:51:51 AM",New customer,tim.jones,CO,1485082311  

So we have a cluster of rows for jim.smith another for tim.jones sorted by the last column in reverse order (i.e. the latest date of the first row is first in the cluster) and a row for john.doe that has only 1 record.

Then I would like to keep only the first row of each cluster. I.e.

"1/30/2017 11:14:55 AM",Valid customer,jim.smith,NY,1485771295    
"1/26/2017 5:06:11 AM",New customer,john.doe,CA,1485403571    
"1/30/2017 11:09:36 AM",New customer,tim.jones,CO,1485770976  

Answer 1

Given file input as

"1/30/2017 11:14:55 AM",Valid customer,jim.smith,NY,1485771295      
"1/26/2017 5:06:11 AM",New customer,john.doe,CA,1485403571  
"1/26/2017 8:02:01 PM",Valid customer,jim.smith,NY,1485457321  
"1/30/2017 4:14:30 AM",New customer,tim.jones,CO,1485746070  
"1/30/2017 11:09:36 AM",New customer,tim.jones,CO,1485770976  
"1/30/2017 11:14:50 AM",Valid customer,jim.smith,NY,1485771290      
"1/22/2017 11:51:51 AM",New customer,tim.jones,CO,1485082311  

then

sort -t, -k3,3 -k5,5rn input | awk -F, '!seen[$3]++'
"1/30/2017 11:14:55 AM",Valid customer,jim.smith,NY,1485771295      
"1/26/2017 5:06:11 AM",New customer,john.doe,CA,1485403571  
"1/30/2017 11:09:36 AM",New customer,tim.jones,CO,1485770976  

Answer 2

With gnu datamash:

datamash -t, -s -f -g 3 max 5 <infile

but note that you need to remove all trailing blanks from each line in your input before running it.
With awk only, using two arrays:

awk -F, '!z[$3]{x[$3]=$0;z[$3]=$5;next}$5>z[$3]{x[$3]=$0}
END{for (i in z){print x[i]}}' infile