Changed to use the Samsung profile (had been avoiding it as I have other Samsung stuff) and this seemed to fix up my double-pressing problems with the FLIRC and the Harmony One. I used the default key delay (which is 0ms, so nice and responsive)
Thanks for the advice !
Consider me in suspense - have been trying to get my Harmony One working with the Flirc, with different TV brands and delay settings, and I'm always getting a "double-press". Better support for this issue would be awesome !