Ideally, like this:

    fs rep gs fs ; Note: extraneous prefix
    mov eax, [fs:ebx]

Some of disassemblers may put them as raw bytes (db 0x63, 0xf3, 0x65, 0x64).

> Note that code can jump past some of the prefixes. The C library on Linux does this to bypass prefixes

What's the use case for this?

I wouldn't split the prefix from the instruction and would rather use label+1 where it's absolutely required.