Knowing the day of the count before saying the blessing:
Lechatchilah one is not to say the blessing over the Sefirah until he verifies which day of the count it is. In the event that one said the blessing prior to knowing which day of the count it is, he has nevertheless fulfilled the blessing and is not required to repeat it prior to verifying and saying that day’s Sefirah. [This however only applies if no interval was made between the verification and the counting and the blessing, such as one said the blessing together with his friend and waited until his friend counted to verify the date and then he counted. If however an invalidating interval took place between the blessing and the Mitzvah, such as he switched houses, or talked of irrelevant matters, then the blessing must be repeated.]
If one is unsure of the date, is it valid to count both days out of doubt, and may a blessing be recited?
Some Poskim rule that an indefinite count is invalid, and hence it is invalid to count two dates out of doubt. Other Poskim however rule that such a count is valid. Practically some Poskim conclude one is to count the two dates without a blessing. According to all, if after counting one verified which of the two dates were correct one is not to recount that day with a blessing. On the future nights, after verifying the definite date, one may continue to count with a blessing.
 489/19; Taz 489/8
The reason: As when one says a blessing one is to know the matter that he is saying the blessing over. [Admur ibid]
 Admur 489/18; Michaber 489/5; Tur in name of Ravayah
 See Admur 489/21; 8/21-22; 432/6
 Avnei Nezer Yoreh Deah 248; Arugas Bosem 168; See Dvar Avraham 1/34; Shaarey Tziyon 5681 3-4/17; Shaarey Yosher Shaar Hasfeikos 5; Keren Ledavid 140; Betzel Hachamah 5/100; Beis Avi 4/65; See Piskeiy Teshuvos 489/17; Nitei Gavriel 35/20
 Igros Kodesh 17/265 [printed in Shulchan Menachem 3/2]; Shealas Shlomo 49; Pekudas Elazer 489; See Dvar Avraham ibid; Minchas Yitzchak 8/48; Kinyan Torah 2/3; Mishmeres Shalom 36/6
 The proof: As the Raza [Baal Hamaor] and Ran explain the reason for not counting two days in the Diaspora is not due to it being invalid to count two dates but due to the mix-up it would cause with Shavuos. [Pekudas Elazar ibid] Alternatively one must conclude that it is valid as otherwise no Jews in the Diaspora ever fulfilled the Mitzvah of Sefiras HaOmer in the times of the Sanhedrin due to Sfeka Deyoma.
 Pekudas Elazer 489; Dvar Avraham ibid; Shealas Shlomo 49; Arugas Bosem 168; See Dvar Avraham 1/34; Shaarey Tziyon 5681 3-4/17; Shaarey Yosher Shaar Hasfeikos 5; Keren Ledavid 140; Betzel Hachamah 5/100; Beis Avi 4/65; See Piskeiy Teshuvos 489/17; Nitei Gavriel 35/20
Other Poskim: The Rebbe ibid and Sheilas Shalom ibid imply one is to count with a blessing.
 The reason: To suspect for those opinions that hold it is invalid, and due to the interval between the two dates and the blessing. [Pekudas Elazar ibid] However see Admur 489/21 that the first mistaken Sefira is not an interval.
 Minchas Yitzchak 8/48
 Nitei Gavriel 35/20; As it is a Sfek Sfeka, as it is disputed in Achronim as to whether one fulfills his obligation with a questionable count.