My players have been reporting when they use /bank it works and sometimes it doesnt do you know what the cause is?

Nope.

Does it block it in a building blocked zone?

By default, you cannot use a bank box if

1. You do not have build permission
2. You're irradiated
3. You're swimming
4. You're flying or falling
5. You're wounded

Could you possibly add a config for notifying if your in a blocked area and you cannot use /bank?
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Also one of my players somehow duped there wholle bank via: opened bank closing it and it started to dupe each time he did it.

There already are messages that indicate if a player cannot open their bank box due to one of those reasons.
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Regarding any dupe bugs, you will need to provide more information.

The player is giving me information now. ill update you on this soon.

https://www.youtube.com/watch?v=D-cIO6tlTOU Bug report - Little confusing
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Someone also just lost all of there  items from bank. i cant be the only one who has this issue

I'm not entirely sure what's going on here.  Is this a modified version of bank?

Not modified