I would think that the WARNING message means that you were trying to enter a single ACL rule (= ACE) that already existed in the ACL.
To my understanding the only way you can have identical ACEs in a single ACL when you have one ACE using a simple permit statement mentioning the IPs/ports in the command and when you have the same done with "object-group". In this situation to my understanding the ASA will actually have 2 identical rules (even though configured differently)
access-list TEST permit tcp host 18.104.22.168 host 22.214.171.124 eq 80
object-group network SOURCE
network-object host 126.96.36.199
object-group network DESTINATION
network-object host 188.8.131.52
access-list TEST permit tcp object-group SOURCE object-group DESTINATION eq 80
When we look at the ACL in opened form we see that the actual rules are identical
access-list TEST; 2 elements; name hash: 0xd37fdb2b
access-list TEST line 1 extended permit tcp host 184.108.40.206 host 220.127.116.11 eq www (hitcnt=0) 0xd82b1952
access-list TEST line 2 extended permit tcp object-group SOURCE object-group DESTINATION eq www 0xbcf2cfe7
access-list TEST line 2 extended permit tcp host 18.104.22.168 host 22.214.171.124 eq www (hitcnt=0) 0xd82b1952
Yet you say that you were moving an previous configuration to the device so it should be valid configuration as it was already used on an ASA.
Are you sure that you have not just copy/pasted same lines again or perhaps used somekind of "show access-list" output as the base of some configuration? That what I was thinking with the above example I mentioned that the access-list output might have identical rules even though the configuration format is different.
I have found a way around this problem, instead of finding duplicates on ASA, I created a little script (.bat) file to find and remove duplicate in notepad, then 'copy tftp running-config' onto the firewall.
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