The first eight bars of the Crucifixus of Zelenka's 1730 Mass (ZWV 13) is almost identical to Handel's Largo dating from 1738. It is in the middle of track 3 of Supraphon's CD 11 0816-2 and on page 66 of the Carus-Verlag edition of the music. Zelenka's Crucifixus is marked Larghetto and is in 3/2 time. What can be the explanation for this? Is it just coincidence?
It's difficult to say whether or not this is coincidence. Handel may have got hold of a copy of all or part of the mass and been inspired by it. Zelenka himself used material from other composers, such as Frescobaldi. Plagiarism possibly wasn't such a big issue in those days...
This could be an interesting topic. Was Handel in Dresden in the 1730s? Did Quantz or Hasse, when they traveled to London, bring with them copies of some of Zelenka's masses? That's unlikely though given strict rules on church music back then. Perhaps the particular theme for the Crucifixus had been written earlier and "recycled" for ZWV13.