have recently faced this issue was getting an error when i was trying to save my changes on a section and it was giving error as "Rule already exists" like below.

i tried by closing the section , then re-open it again tried to save it after changes but it was giving me the same error.

so i noticed that in my check out rules there were multiple instances of same section that i have done (probably when i have checked out them and tried to save they had not saved during that time as i have directly closed the window as save button was not getting clicked) , you can check checked out rules like below  

so i discarded all the checked out rules for this same section and opened fresh window and made change on this section and saved it worked.

Note- mostly this issue happens for Private edits and checked out rules that you were unable to Save or check in when the pop up has arrived and Save button is not clickable and you close the window for that rule.