Duplicates will be deleted, very old threads that are re-opened (necro'd, as in raised from the dead) are sometimes locked.
If you are so foolhardy that you add a new post to a thread that is five years old, and not touched since, and the new information is in no way relevant to the old situation, then there is a chance I will lock the thread, and delete the new info (which is worthless anyway).
new information should be posted in a new thread, otherwise people will get hopelessly confused by the obsolete information at the start of the necro'd thread.
but yes, duplicates will be deleted without warning, moderators always will give a reason for a removal, if only the word "duplicate", but I don't know if that information reaches the posters. But please do know that one of the "rules" here is that moderators will delete duplicate posts.
If something happens to your post, it might be time to read your warnings, one unfortunate problem is that with the removal of personal messages there are no longer automatic e-mails sent when you are warned, if you continue doing bad things after being warned several times, I will give you a short (few days) ban to "get your attention", and will say in the ban to read your warnings. But it might be wise to read your warnings if posts are suddenly removed.
the literal only "taboo subjects" here are "where can I find illegal ROM sites", "how do I do malicious hacking", and the infamous "FM transmitter" which is in fact a broadband (square wave generator) emitter. IR tranceivers is NOT on my naughty list, but messages that seem "nonsensical" are.