Yup, that's the way it works.
When I do it (double post), I cut and paste the new stuff from the second to the first using edit. Then I edit the second to "duplicate, please delete" or similar then report myself (using the "Report" tab at the bottom") to alert the Mods (who are the only ones that can delete it).
That usually happens very quickly, if a Mod's on duty.
Screwing up doesn't make me popular, but I'm told that's the preferred technique?
OF
I'm not seeing these edit options ( "duplicate, please delete" or similar...") in the Edit window. I hope not to tax the mod too much with my fumbling.