Was doubting Thomas at the first Pentecost?

No, the first Pentecost was well before the time of the disciples. However, if you mean the first Pentecost after the Resurrection, then yes, Thomas was with them, although no longer doubting.

13And when they were come in, they went up into an upper room, where abode both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James. 14These all continued with one accord in prayer and supplication, with the women, and Mary the mother of Jesus, and with his brethren.

And when the day of Pentecost was fully come, they were all with one accord in one place.