Objective is to rename photos eg IMG_1234.jpg to 2014-02-15 14.31.23.jpg - no problem with custom autodate - until there is more than one photo with the same timestamp - common if using burst shots or merging photos from more than one source. Its then necessary to do another pass with a suffix e.g. -1 added to the custom format field , select the files that "errored" and redo. keep going for further errors....
That's cludgy - suggested new functionality is to add the suffix automatically when duplicate New names are found.
Unless I have missed some really obvious existing functionality.