This happens because the default name of the role is already "@everyone", so when you retrieve the mention it adds another "@" to it, becoming @@everyone".
What you could do to solve this would be replacing this with "everyone" like this:
n = [role.mention for role in ctx.guild.roles[1:]] # Ignore the @everyone
n.insert(0,'@everyone') # Add string @everyone at the beginning
# Here goes the rest of your code
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