If there are two strings on the same topic in an area that you are moderating, join both of them into one string. DO NOT just lock one, rather use the merge button at the bottom leftof the board and merge the two.
Besure that you merge the newer string into the older string. You can tell with one is older by the number associated with each string. Low number means older string, higher number means newer string.
If you have questions please ask.