No, it's the first two bits of every instruction (RISC-V is little-endian, so these are the least-significant bits). Two bits have four possible values, three of them are for 16-bit instructions, one of them is for 32-bit instructions.

> So a 32-bit instruction has this information duplicated in the same place in the latter 16-bit half, since a decoder has to be able to decide whether it's trying to decode a 16-bit instruction or whether it's in the middle of a 32-bit instruction.

No, that information is not duplicated. The decoder cannot know whether it's in the middle of a 32-bit instruction or not; it has to decode the length of all preceding instructions. That's why it's important that you can know the instruction length without decoding the instruction, so that simple logic can tell decoders other than the first whether they're in the start or in the middle of an instruction.