Raise error for non repeated fields being listed twice in TextFormat

[thomasvl]

Looking at the C++ code:

https://github.com/protocolbuffers/protobuf/blob/6051341ab21b4e1f75e91710ab1a8cc8fadda521/src/google/protobuf/text_format.cc#L536-L540

They error if a field is listed twice in TextFormat if it wasn't a repeated field. The code for oneofs is right near it, but Swift doesn't seem to have this support.

Quickly skimming the controls, there is a toggle for this, but it seems like an internal impl detail that can't be shut off from the public api.