My test case is a DUAL lister with a single folder called Source selected in the left pane and Source.zip in the right pane. Both Copy ARCHIVE and Copy ARCHIVE=SINGLE pop up the Merge, Replace, Cancel prompt but behave differently if I select MERGE.
Copy ARCHIVE + MERGE does its thing with no further prompting whereas Copy ARCHIVE=SINGLE + MERGE prompts me for each file that exists in Source.zip.
Further testing shows that when there is more than one selected folder (left pane) and existing folder.zip archive (right pane) and I execute Copy ARCHIVE=SINGLE then each folder generates a Merge, Replace, Cancel prompt but after answering MERGE, only the first folder prompts to replace each existing file. The remaining folders are merged quietly.