Opposite/inverse of duplicate

Not sure if this helps, but original has the following senses.

• Preceding all others in time; first.
• Being the source from which a copy, reproduction, or translation is made.

You say original comes with the notion of "designed similarity", but perhaps we can call a post "original" if it simply precedes the newer one in time.

Extending a metaphor a bit, you might use touchstone. If you put it in quotation marks the first time and explain it, then it should be O.K. to use for your purpose.

EDIT: From a purely technical point of view, ur-Question would be just right... if it weren't a candidate for "world's ugliest word" :)