public class AvoidduplicateAccount{

public void AvoidDupliacteRecords(Account[] acc)
{
set<string> set1=new set<string>();
for(account a:[select id,name from Account])
{
set1.add(a.name);
}

for(Account d:Trigger.new)
{
if(set1.contains(d.name))
{
d.adderror('plz insert unique values');
}
}
}
}