No, they'd still have the expected values.
For one thing mIRC's scripting language is single-threaded so only one alias/event/whatever is processed at any given time, meaning if someone joins while your code is looping the join event code won't be triggered until after the loop has finished and the code block it's in has ended.
Even if mIRC scripting were multi-threaded it wouldn't make any difference (in theory) as long as they were local variables in which case they're effectively different variables for each scope in which they're used.
