Union advised me that if I'm written up for anything, I don't have to sign anything they ask me to. Is that true?

I haven't been written up but it's occurred to me I should ask just for clarity's sake.

I'm a little unclear on this and my union rep didn't make clear whether there are any potential repurcussions, but if I refuse to sign a written warning, can't they fire me? I always assumed so.

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