As was observed (by others) under related q's here on material from that site, it seems Pfzier/BioNTech has done a global optimization to increase CG contents in the sequence, generally thought to be beneficial to mRNA expression (in addition to all the other things they've done, like U -> m1Ψ global substitution, specific to mRNA therapeutics).
That (global CG target) may also explain why they replaced UAA with UGA, even though the latter is actually a "worse" stop codon, in terms of allowing more read-through. As one paper (preprint) notes:
despite having the lowest efficiency of translation termination among the three, UGA is the most common stop codon in many multicellular eukaryotes. The relative frequencies of the three stop codons are dependent on multiple factors and strongly associated with regional variation in GC content. The frequency of UAA is strongly negatively correlated with GC content, while the use of UAG and particularly of UGA increases with increasing GC content.
Using two UGAs may be a way to compensate for this choice (of using the lowest efficiency stop codon).
Actually, one more complex theory is that the stop thing is actually a tetranucleotide:
In both bacteria and eukaryotes, the base immediately following the STOP codon (position +4, with the first nucleotide of the termination codon marked as +1) exerts the strongest influence on RT (readthrough) efficiency. This has led to the hypothesis that an actual translation termination signal consists of a tetranucleotide sequence, not only the STOP codon itself. Which tetranucleotide is the most efficient in eliciting the RT in eukaryotes remains controversial. [...] For example, the level of basal UGA-C readthrough in mammalian cells (3–4%) was shown to be 3–6 times higher than for the remaining UGA-N tetranucleotides.
If we go by this, then UGAU is the stop tetranucleotide here, which is perhaps better than just UGA, especially if see that a C follows the 2nd UGA in the actual WHO doc, so that might have tripled (or more) the read-through chance if the UGA were not doubled before it:
ΨGA ΨGA CΨC ...
Aside: The CΨC begins... a "double UTR" (dUTR) as they call it (even though this one is composed of two different UTRs) for which there is a published paper that it is more efficient (in terms of stability etc.) than a single UTR. So, generally speaking, the doubled stop seems part of a strategy to "double everything" in that terminal region if it helps with translation efficiency.