Well, there's actually some debate as to exactly how the problem manifests, but as I understand it, you cannot retrofit more than one design per location per turn. (Location = sector with shipyard, AFAICT, but it might just be shipyard.) There's disagreement over whether you can retrofit multiple ships to the same design at one location. I've been testing different combinations as I play, and so far I have successfully retrofitted two identical ships at two different shipyards in the same system in the same turn.
Search the forums for the words "retrofit simultaneous" for more details. [Ignore the 21 page thread about the Pirate mod.
]
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Cap'n Q
The most merciful thing in the world, I think, is the inability of the
human mind to correlate all of its contents. We live on a placid
island of ignorance in the midst of black seas of infinity, and it was
not meant that we should go far. -- HP Lovecraft