Error 'System.ListException: Duplicate id in list' in Apex

1. Create a map of <id,sobject>
2. Convert the list to Map so that the duplicate IDs are removed.
3. Update or Delete the values part of the Map.

Sample code 

// pick up any id from your salesforce org, in this sample it is account id. 
id aid = '0017F000002WkkdQAC';

list <account> al = new list <account>();

for(account a : [select id from account where id ='0017F000002WkkdQAC']){
	account acc = new account(id = aid);
    al.add(a);
    al.add(acc);
}
//create a map that will hold the values of the list 
map<id,account> accmap = new map<id,account>();

//put all the values from the list to map. 
accmap.putall(al);
if(accmap.size()>0){
update accmap.values();
}