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Suppose I say mincha. Later the same day, forgetting I'd prayed already, I say mincha again. This second prayer said is after the earliest time one can possibly say maariv, possibly even after sunset. Suppose I realize my error after finishing the amida (sh'mone esre). Does the amida count for maariv?
(Assume this isn't Shabas into motzae Shabas, or any other day where the mincha and subsequent maariv differ substantially.)
The Shulchan Aruch and Mishne Berurah (107:3) write that if one is davening and remembers that he already davened he must immediately stop because tefilos correspond to the korban tamid and by davening twice it id's equivalent to bringing two korbanos which is forbidden. Clearly, the berachos of tefilah in such a case are all in error and sadly are berachos levatala.
If after completing the tefilah he became aware that he had already davened, his berachos were also levatalah for the same reason i.e. you may not bring two korbon tamid.
Since the tefilah was invalid it cannot count for any tefilah. It is evident from 108:1 and MB 7 that the intent at the time of davening for a particular tefilah is fixed and can't be changed after davening.
