Cell Name Collision
When merging two or more GDSII files there is a possibility that a cell (structure) name could be used in more than one of the files and that the contents of the commonly named cells differ. The merged file would not be correct.
Either the cell ADD1 will come from File A or it will come from File B but the merged results will be incorrect.
The Overlay tool avoids this problem by appending to each cell name _N, where N is the merge order. So the first file in the merged list will have its cell names appended with _1, the second file in the list with _2 and so on.